Elites is a really tough slot because of one unit and one unit alone: Trueborn. They bring such incredible anti-tank and anti-infantry potential that everything else in the slot has to be matched up against them when building an army. When not taking them you have to consider the opportunity cost of doing so. That being said Dark Eldar do have some excellent choices in Elites and since so much anti-tank and anti-infantry potential can be found in every Force Organisation slot in the Dark Eldar codex, Trueborn aren't an auto-include nor the only good choice. Let's look at what else we have.
Since there are so many Elites we have to cover we've split this review into two parts. Part 1 looks at Incubi, Gortesques and Mandrakes whilst Part 2 will look at Trueborn, Bloodbrides and Harlequins. Wracks aren't covered. Never ever ever take them as Elites. There is literally no reason. Jeez. They are Troops.
Incubi
The good version of Banshees. High natural Initiative means they strike ahead of anything except other Eldar and special characters and S4 power weapons mean they chop up most things pretty effectively, especially with two attacks each. They're also tough enough to shrug off a fair number of attacks, but remember that T3 makes them more vulnerable to just being torrented than Marines are, though a pain token mitigates this a lot. They are pricey and with T3 and no invulnerable save aren't going to survive protracted combats or be the rock your army is looking for. Do NOT send them into a fight where they can be engaged by a Dreadnought or Monstrous Creature, as they have no real way to handle them.
Due to their high number of attacks, WS and initiative 5, large squads of them (more than five or six) is probably overkill against most targets. This is the trick to getting the most out of this unit, managing unit size. Too large is going to generate a ton of overkill which generates inefficiency and leaves you out high and dry after most combats. You can however lose a couple Incubi and still be effective. Smaller squads are less likely to suffer overkill but rather stay locked in combat and protected against shooting but losing a couple members can devastate the squads ability to damage opponents. You then also have to consider the addition of an Archon who is generally a combat beast as well. Archons add the very important Phantasm Grenade Launcher which allows Incubi to assault into cover and not get punched in the face before they can swing. In the end squad sizes of around 5-6 + potential Archons are a good middle ground that doesn't break the bank. Obviously these guys also need a transport (Raider) and if they can get an early pain token or two, will just start ripping through units and be pretty durable to boot.
The Klaivex and his various upgrades are just not really worth it, as none of them really bring anything new to the unit and are less efficient than just adding another body. Remember Incubi have Fleet which makes them highly mobile and very capable of getting assaults off out of their transports.
If you're looking for a combat squad which packs a hard punch and is going to kill units rather than wearing them down or tarpitting (such as Wyches) or torrenting/Rending (such as Beastmasters) than Incubi are your guys. They aren't cheap and will be a big target to your opponent early so getting them early FNP is very important or overwhelming your opponent with threatening targets. No ranged options or any anti-tank hurts them as they are very uni-dimensional but they do that role very well.
Grotesques
Often overlooked, Grotestues are actually incredibly difficult to bring down with most guns, as T5/W3/FNP is very annoying. They have a crappy save (6+) so cover is their friend and unfortunately this slows them down. They aren't fleet and don't have move through cover and take up extra space in Transports so you aren't going to be carrying around large squads. Webway Portals can help with their movement restrictions but even then, no Fleet hampers them. The biggest drawback for Grotesques though is requiring an Independent Character to 'control' them and make sure they don't go nuts. This is an annoying little tax you have to include into your army list and can limit your ability to move pain tokens amongst units. That being said, without a character the terrible leadership of Grotesques would make them a useless unit.
Grotesques put out pretty respectable damage, although nothing totally overwhelming; mostly, they are incredibly hard to take out and rely on grinding the opponent down. Furious Charge raises their strength an impressive 6. Grotesques really start to shine when you add Urien to the army, however, as you can make them +1Str for +5pts, which is very much worth it. S7 on the charge and chance of even more pain tokens? Yessir, I think I like it. Urien overall isn't pretty hot as covered before though it really is the only use for him. Adding other characters such as a Succubus or Archon to give this unit reliably killing power kills two birds with one stone but you're really looking for this unit to absorb a lot of firepower before going down.
Aberration upgrade isn't worth it, as it takes your Ld from "abominable" to "horrible" and none of the CC weapons are actually particularly useful (power weapons; please?). The Liquifier is nice, though. Taking a dedicated transport is sort of awkward, as you can only fit four of them (and an IC) at max into a Raider but as discussed before, they are slow as hell without some sort of a ride. Using them in a Webway army + Urien is probably the best way to run them though a couple squads as hard units to take down which can also punch crap in combat in Raiders works well also. They are expensive however (more-so than Incubi) so large squads are prohibitive points wiseand small squads don't really have the amount of attacks to scare people but rather just annoy them so fitting them into your army can be difficult.
Mandrakes
Much maligned, actually not that bad; once they have a pain token they are a force to be reckoned with. Getting that pain token however can be annoying. You can start a Haemonculous with them (which means no Infiltrating; no thank) or jet one forward in a transport and deposit it such that they meet up. A Chronos passing them one is also not impossible or simply using ICs to move pain tokens around your army as usual. Once they have their token... they're kinda beastly. 3+ cover, 5++ (for Flamers/CC) and FNP, making them incredibly difficult to bring down. Moreover, their blast is pretty decent and they can charge in after using it- they're S4 naturally, so they can put out reasonably decent damage. The keys are getting them that pain token and making sure you utilize BOTH their melee and shooting abilities.One of the main reasons to bring Mandrakes however is infiltrate. In an army full of mobility and unprecedented fire and move capacity, there is very little scout and infiltrate based units. Mandrakes fit this role perfectly, able to deploy ahead of the army and due to their decent ability to both shoot and assault, delay and disrupt opponents. They really do need that pain token though to be really annoying and two obviously heightens this.
They aren't too expensive at just 15 points a model so a medium sized squad (5-7) is pretty cheap and adds some disruption and defenses. Problem is you can't take a transport with them which lowers your vehicle count and ability to max the FoC but does something in the army nothing else does (infiltrate). The Nightfiend really has no point outside of upgrading Leadership for Mandrakes. It's a nice addition, particularly against tank shocks, but has no options himself to make the unit more deadly in combat.
Overall, Mandrakes play a very specific role through the ability to infiltrate. Their shooting is pretty decent and backed up by assault can finish a lot of units off but are really an annoyance to your opponent which delays them getting to your lines or disrupts theirs. If you don't want/need anything like this or don't have enough anti-tank in the rest of the army, don't go Mandrakes. They are probably more viable in a foot based list, whether it's Hybrid or not, due to the lack of transport options as well.
Conclusions
The Incubi are the clear winners here as they do what they do so well. Grotesques really need Urien to be viable for S7 on the charge and even then, need IC babysitters and more than likely a Webway Portal (and some better units can come out of the Webway than them in terms of combat potential). Whilst they do inflict wounds, getting the model numbers to inflict a lot of wounds is very pricey though they do absorb damage quite well with FNP and cover. Realistically, these guys should be left home as they are just too fiddly though if you are desperate to take them, look to an Urien/Webway based list. Mandrakes are not as bad as many people have envisioned as they do something unique in the codex (infiltrate) but without a pain token, are pretty expensive for a utility/disruptive unit. With that pain token though they become a very viable disruptive unit but lack the ability to take a transport.
Overall, none of these choices can really fulfil dualistic roles. Grotesques and Incubi can do so with their transports to an extent but in the end, these units have specialised roles and are taken to complete them. The rest of you army then has to shore up any lose in anti-tank 'lost' in this way.
