The Necron codex actually does something very interesting with its special characters. Unlike many of the recent books, there are no FoC-swap characters, be they generic or unique, anywhere in the book- in fact, no FoC-swaps of any kind, which I think is actually the only book of this edition with that distinction. (SoB don't count.)
The special characters do, however, open up unique builds of the army, and they do so by virtue of their special rules and characteristics rather than by an explicit change to other units. While there are some characters that do this elsewhere, I think Necrons are perhaps the most effective example of GW having done this.
Imotekh the Stormlord
Let's get a couple things out of the way: Imotekh is not an auto-include, even at the point values he's viable at, nor is he appropriate for every- or even most- armies. Like all of the other special characters, he offers you a unique suite of options to build a different type of army.
What options? Well, he has a perfectly normal statline and can take a Command Barge, but given that he doesn't have a Warscythe there's really no reason to do so. He has excellent defenses (2+/3++, Phylactery) and a one-shot weapon that can do some okay damage to infantry. He's one of the few SCs that are a Phaeron, which can have uses, and Seizes on a 4+ (except against Orks.) He also has some garbage about winning combats and Flayed Ones, but neither of those rules should ever, ever come up. His big deal is causing semi-continuous Night Fighting (which you will want to augment with a Chronometron) and random lightning strikes so long as it continues. The strikes are mediocre against infantry but quite good, if unpredictable, against vehicles, since they are S8 and hit side armor. They tend to punish MSU the most, but will probably do some damage to just about any army you face as a "bonus" on top of why you took him.
So you can cause Night Fighting for a while. Okay, that's nice- regular Necrons can get two turns of darkness anyways without shelling out for a special character- remember, Imotekh costs as much as TWO Scythelords by his lonesome, so you need to get some serious mileage out of him. Can you just stick him inside any ol' 2000pt list and let him do his thing in place of the Solar Pulses? Absolutely not. Solar Pulse only applies on the enemy turn- Imotekh's ability is continuous and will hurt you as well. Necrons may tend to be a bit shorter-range than their opponents, in the 24" band, but at that distance you are still losing nearly half your shots to being unable to see. Sure, the enemy is as well, but they didn't have to pay points for the privilege of doing so.
So if you're going to use Imotekh, you need to take advantage of and mitigate his ability at the same time. Note that Solar Pulse won't help you any here- if you shut off Night Fight on your turn, you won't get the lightning strikes. That doesn't mean you shouldn't take them, but don't count on them to make things work out. You also need to be making good use of what he does- that is, making sure the units you take are ones that need the protection from shooting that Imotekh offers.
What units benefit from this? Melee units especially- Wraiths, Scarabs, Spyders, and Lychguard all appreciate the reduction in incoming firepower that he can bring. Night and Doom Scythes also find it useful, as they (unlike most Necron units) are fast-moving and can change ranges quickly to confound the enemy. The Doom Scythe also has the advantage of its main gun not needing to roll for Night Fight because it does not actually target a specific unit.
Is Imotekh good? I think he is, yes- the ability to punish shooting and MSU strategies, two major foundations of the 5E tournament scene, is quite powerful. However, he relies on a potentially-awkward build and a few poor dice rolls can leave the whole army sitting in the sunshine to get shot to pieces. You can very consistently get two turns of cover, but more than that can be tricky- for that reason, bringing a Solar Pulse is still going to be required for such an army. Note also that many armies (Tau, SW, DE) can mitigate the effects of the limited visibility and virtually all Imperial armies have the option to slap a Searchlight on top as well, though not all of them get it for free.
Nemesor Zandrekh
Coming in cheaper than Imotekh, if not by a huge amount- he's still a pretty pricey dude. Like a lot of the Overlord characters, he comes with 2+/3++ and he also carries a Rez Orb, which adds some nice utility. Once again we have the option for a Command Barge, but without a Scythe it's pretty useless.
His unique rules are interesting, but somewhat unimpressive all things considered. If you hold units in reserve and the enemy does as well, you can "respond" to their arrival by Deep Striking your own units; unfortunately, this won't actually be terribly helpful, since you'd be showing up just in time to get shot, and Zandrekh himself has to be on the board for it to work. So a cute trick for Dawn of War maybe, but that's about it. His other two rules are much more useful- you can pick one unit in his LOS on the enemy side to lose most of its USRs (but notably not Move Through Cover, Fleet, or FNP) and may likewise choose one unit on your side to gain a USR (with Tank Hunters, Night Vision, Stealth, and Furious Charge all being reasonably good picks.) Remember that due to the FAQ you can't take away abilities from inside a transport, since they require LOS.
The question is: how worthwhile is this compared to a normal Overlord? He still brings a Court along, so there's no trouble in that regard, but with no Phaeron or Warscythe he lacks an obvious role to fill by his numbers alone. Taking away rules from the enemy is interesting, but there's no guarantee they'll even have any of those rules to remove, or that doing so will be relevant. Stealth (for breaking fragile cover-based units) and Night Vision (to make mitigating Solar Pulse harder) are the only two real standouts, though others could sometimes be useful. Selectively applying USRs to your own unit, on the other hand, is more useful and something that can be planned around, but it has difficulties of its own. Necrons are oddly lacking in good targets for giving USRs; for example, Tank Hunters wants a unit that is carrying multiple heavy (or at least mid-strength) weapons, but many of the "best" choices there are Tesla, which is actually not very good for killing tanks due to AP-. A lot of people get excited about Furious Charge, generally on Lychguard with Scythes (for S8 and IDing characters) or Scarabs (to wound/penetrate more easily and strike ahead of Orks), but I don't think either option is all that amazing. S8 will only really matter against characters, who should get murdered by your flurry of high-Str attacks anyways; with Scarabs, Entropic is already ensuring that you murder tanks to death easily, so FC only really helps with trying to push an Entropic wound through onto some T4/2+ type guy, which isn't really what Scarabs are for anyways.
All in all, Zandrekh is not a horrible buy and he does open up the ability to take advantage of your units in interesting ways and subtly shift the battlefield in ways that can really make a difference, but he's going to require some very deft play to use.
Varguard Obyron
Zandrekh's bodyguard, Obyron is NOT an Overlord- in fact, he's basically just a souped-up Lord with IC status. His stats are pretty nice- he's the only Necron who goes above WS4 and he does come with a 2+ save, but doesn't have an invuln- which is a big strike against a combat character. He has a Scythe and a Veil that can be used to teleport out of combat, but seeing as he's supposed to be a combat guy the ability to run from fights is not all that great. His special rules let (force) him to teleport to anywhere that Zandrekh is being attacked and join the fight and also give him free attacks equal to the number of swings that missed him, so long as they happened before his own attacks (maxing at six bonus ones.) The latter goes a long ways towards mitigating his I2 and can make mediocre combat units (one-attack Marines, IG, etc) pretty miserable.
The problem, however, is that Obyron isn't cheap and doesn't come with a Court, so you're sinking a bunch of points for a punchy kinda guy when an Overlord can do a very similar thing for a better price and better utility. Pairing him up with Zandrekh (he doesn't take a FoC slot if taken with his buddy) is only going to be workable in a really large game, and even then I think it's unimpressive. Obyron is far from the worst of the special characters, but lacking an invuln, a Court, or a particularly good price/effectiveness, he's just not going to make the cut a lot of the time. His teleport shenanigans are amusing, but as with Crypteks, at the end of the day they don't accomplish all that much.
Illuminor Szeras
This guy is seriously the bottom of the barrel and I almost can't imagine a situation where I'd want to take him- he's really that bad and generic. Remember what I said about how most of the Necron characters are interesting because of how their rules interact with the rest of the army? This guy is why that statement uses the word "most." First of all, he's a wannabe-Cryptek with a better statline (W2, 3+ save and A4, for some godforsaken reason.) He's an IC, but like several of the others, he doesn't get a Court- as always, this is a major strike against using him. He comes with the usual Harbinger of Destruction Eldritch Lance and has Gaze of Flame as well, but no Pulse; boo-urns! He also buffs up one unit of Necron Warriors to be either T5, S5, or BS5. Two of those are pretty good; one of them is shit-worthless.
So he has a very middling suite of abilities and gear, but if he was cheap it might be defendable- a reasonable way to bring an extra Lance and some stat buffs for a small price. But no, you're paying 100pts for this garbage and losing out on the opportunity to take someone useful instead. Even in small games an Overlord or Destroyer Lord is much more useful for a tiny fraction more points. Szeras is completely worthless.
Anrakyr the Traveller
Anrakyr is interesting because he is a straight upgrade from the "usual" Bargelord- he carries a Warscythe and Tachyon Arrow and can get a Barge just like normal, but for 35pts extra. What does that get you? It gets one of your Immortal squads FC and Counterattack, which is not interesting but is a pure upgrade and so might theoretically do you some good. He himself likewise has those USRs and most of the time can "take over" one tank within his LOS- unfortunately due to the FAQ this means he can't use the ability from within a Barge, which is sad.
If you have some points to spare, Anrakyr is well worth considering for many Necron armies; even if you have to finagle it a bit, sometimes getting to shoot with the enemy's tank can be pretty devastating. He is probably the most "generic" of the special characters, but in this case that works in his favor, as he has good gear and no obnoxious rules to drive his cost up too high.
Orikan the Diviner
This is the other guy who's gotten a lot of press lately, mostly in combination with some sort of stupid Writhing Worldscape/Tremorstave gimmick list. I'll straight-up say it: those lists are bad and mostly won't work. Orikan doesn't give you a Court, so you're sinking for him AND an Overlord AND an expensive, vulnerable MC. That's an easy 500pts down the drain and you haven't even purchased a Court or Troops or anything yet.
With that out of the way, Orikan is surprisingly well-packed with abilities. Everyone knows about making the board DT first turn, which can be helpful in pinning the enemy in place or ruining things for them in Dawn of War, but he also gives you a once-per-game reroll on all reserves for a turn- a nice little boost in utility, even if his other abilities don't particularly mesh with it. Orikan himself has a nice statline and a great weapon- rerolling misses and ignoring saves together is a pretty strong combination, even if his statline is a little weak for actually using it. Note that he may only have a 4+ armor save, but he has a 3+ invulnerable save, which is pretty huge once he transforms. He carries a Transdimensional Beamer, but don't expect to actually use it all that often, nor for it to do much when you do fire it.
His transformation is an interesting add-on, effectively making him into a C'tan for a period of time (but retaining his IC status.) Since he's toting a 3++ and rerolls misses in combat, he's only missing the extra d6 penetration- but S7 is plenty enough for most purposes. Like Imotekh, you really NEED to be bringing a Chronometron along with him to ensure that he switches forms when you want him to and doesn't immediately change back- a very real possibility when you're throwing the dice unaided.
What is he good for? Well, he can go a long ways towards bringing melee defense to a Necron list for a reasonable price, although the unreliability of it can be a problem- but if he strikes at the right moment, he is going to cause some real havoc to most anything short of TH/SS (and even they are not going to tear through him.) His ability to lock down vehicles in the first turn gives some potential for a Scarab alpha strike- if you're going first, the 24"+2d6" threat range can potentially be hitting a lot of tanks that remained stationary. If you can find a good reserve build his other ability could be useful, but I don't think Necrons have the pieces to do it- rather, it will help make sure that holding a few units in reserve (in Spearhead, etc) is still a reliable way to bring them onto the table.
Is there a build for him? Obviously yes, although I don't think it can stand up to other strong lists, it still has at least some potential. He might be able to fit into some of the large-points Tremorstave lists, as he interacts nicely with the Scarabs they otherwise want to run and can serve as the point defense that cuts off an enemy unit that manages to roll well or otherwise escape your attempts to cripple it.
Trazyn the Infinite
I do so love Trazyn as one of the coolest bros in the whole codex when it comes to fluff, but his rules fall a bit- if not excessively- short of what you'd usually like to see for the price. In fact, his list of rules and gear is rather short- he's a Phaeron, which is actually somewhat awkward to use since you often won't want him hanging out with squads of Warriors or Immortals in order to properly use his other abilities. He gets Mindshackle Scarabs, which syncs nicely with some of his other stuff, and a special weapon that "echoes" wounds to other models of the same name as one you killed. He can get a Barge and his weapon gives him marginally more reason to do so, but in the end it's probably not a great buy.
More interestingly he is extra-resilient compared to the usual HQ choices; he he dies (i.e. fails to come back with Everliving), on a 2+ he will "possess" one of your other models (Lychguard, Cryptek, Lord, or Overlord) on the field and take their place. Just as importantly, Trazyn is a scoring model and can thus significantly change the purpose of some squads just by being around, turning them from "ignore and avoid" to "must remove from objective."
Unfortunately, Trazyn himself does not actually help any there, since he contributes almost nothing except the scoring status. Compounding this, your Crypteks and Lords will usually be hanging out with a troop squad anyways, making Trazyn's replacing them somewhat unimpressive or even harmful. His best path, then, is to get into a squad of Lychguard or Deathmarks, most likely the former because of the randomness of where he appears, and make the unit a hard-to-shift mass that doesn't die easily at range or up close. This ends up being a pretty expensive way to add a scoring unit, but it can bring some unique qualities (countercharge, anti-horde) to a Necron list and really is quite annoying to actually bring down.
That completes the rather length review of the various HQ units; we'll move on to Troops (and dedicated transports) next, which will be a bit more brief, before tackling the other FoCs, as the TR slot is the other big defining choice for which direction a Necron list points.
