Fast Attack is in kind of a weird place for the Dark Eldar; it's got some anti-tank, but it's also got some good melee which can be found other places to varying degrees. It does not, however, have a single vehicle of any kind, which can cause problems if you're looking for saturation. Fast Attack is generally not a slot you build your force around and thus one of the last slots to finalise in a list. This is exemplified as it is one of the places you have the most flexibility in choices, along with Elites. However, it's very important to remember it is one of the few places you can access melta type weapons and importantly, they are on excellent platforms. Before we start we’ll preface this by referring back to the Scourge and Reaver comparison and reminding everyone that Hellions should only be taken as Troops with the Baron. Let’s take a look at our remaining options then!
Scourges
Although they can put out some pretty respectable anti-infantry shooting with shardcarbines, the real reason you want these guys is the ability to cram a bunch of AT weapons onto them (2 per 5 models). Although most of the Dark Eldar army is fast and mobile enough to deliver anti-tank where it is needed, Scourges don't rely on a transport for this so are always going to move 12". This is great for weapons like Heat Lances and Haywire blasters which can be fired on the move.
In this respect Heat Lances are often the superior choice but Dark Lances are not actually bad either- being able to get to superior firing positions with their mobility is actually quite useful sometimes. Haywire Blasters are also excellent guns capable of stripping tanks of their ability to function but aren't exactly reliable in destroying tanks.
Blasters and Splinter Cannons aren't really good choices. Blasters come everywhere and often in more frequency (Trueborn) and splinter firepower is prevalent throughout the army. Shredders are also quite readily available through Warriors and as discussed in their review, aren't too flash but can help encourage your opponent to spread out. Solarite upgrade is worthless, don't bother as they shouldn't be in CC regardless though the Ld9 is nice against shooting deaths but generally not worth it.
Scourges in the end are mobile fire support with some utility. Having shardcarbines helps their anti-infantry power, especially when you get a full squad of 10. However, they are pretty fragile with T3/4+ and don't like landing in cover to ensure they get that 4+ cover save. They are in direct competition with Reavers as well in terms of weapon layout which the comparison post liked in the first paragraph covers more in-depth.
Reaver Jetbikes
Having the option to both Turbo-boost or use your Eldar Jetbike scoot is very handy and gives them unprecedented maneuverability, which mitigates the range on their guns a lot. Heat Lances are generally preferable to Blasters, as you can get the latter all over your list but the former only in Fast Attack slots. The range of the Heat Lances is particularly benefited by JSJ as you can ensure the Reavers aren't exposed after they pop a tank. Three Reavers simply isn't reliable enough- with six being the sweet spot and nine being a more combat oriented choice (particularly with Duke).
Their drive-bys are a relatively unique ability to cause damage when they Turbo-boost. This is a nice bonus but isn't going to cause a massive amount of damage. The Arena Champion however is an upgrade to consider, particularly if you are running larger squads or the Duke. Ld8 is always annoying so the Ld9 helps against shooting deaths and in combat situations whilst a combat weapon + combat drugs really helps Reavers not just tie up backfield units but win combats reliably.
In the end Reavers are going to backfield harassment and utility units. They bring some excellent anti-tank to the table in Heat Lances and with their decent combat ability, drive-bys and splinter rifles have decent anti-infantry ability as well. Unlike Scourges these guys are going to be in the thick of it using their special abilities and statlines to ensure their survivability whilst being a royal pain to your opponents.
Beastmasters
Seeeeeexy. One of the better CC units overall, in many ways comparable with Incubi- they have much better anti-horde applications but can still do a reasonable job of cutting down MEQs and whacking at tanks with the right setup. Beastmasters are shockingly, beasts, so have the excellent charge range of 19-24". This is fantastic out of a webway portal in midfield and even starting on your own board edge will get you across the field quickly. Move through cover allows them to take maximum advantage of terrain to increase their survivability but they don't have grenades. Attaching an Archon with a PGL if within a 6" charge range could help reduce this issue.
What's really interesting with this unit is the potential combination of units you can have due to three Beast options and the Beastmaster himself. To get the most from this unit you are going to have to combine different models to ensure the unit doesn't have any glaring weakneses. Let's take a look at each one separately.
Razorwings
Razorwings are awesome, with a big pile of wounds and Rending on their attacks but are only a paltry strength three. This will lead them to have some problems with T4 hordes (i.e. Orks) where there are a lot of bodies and Rending doesn't matter. They are also vulnerable to MCs and Power Fists or anything which is S6 in combat due to instant death. Low save means they aren't that durable either but five wounds certainly helps.
Khymeare
Khymerae are a good way to shrug off PFist wounds or other big, slow hitters or just make the unit more survivable overall with a 4++. They also put out pretty respectable damage for their points which is importantly at strength four. They are also a different initiative value from Razorwings and Clawed Fiends which ensures Rending hits are wasted due to wound allocation. These guys are best taken to increase the survivability of the unit in combat and suck up hits from your heavy hitters.
Clawed Fiends
Clawed Fiends are T5, which is generally irrelevant except for purposes of instant death. You are very unlikely to ever get a majority unit of Clawed Fiends since you can only take one per Beastmaster and this means the T5 is generally wasted. As mentioned before however, T5 can suck up S6/8 wounds and not die to instant death which is a very nice bonus to have. Clawed Fiends also get extra attacks as they take wounds, which is a nice way to take advantage of sloughing wounds onto them. These guys are quiet expensive though so you'd only want to look at taking one or two guys to take advantage of their high strength and toughness.
Beastmaster
Venom Blade is the only weapon upgrade to bother with for the squad, and even that isn't a given; more bodies is almost always nicer. The Beastmaters just aren't that hot in combat and are there for Ld purposes (8 compared to 5).
Mixing Khymerae/Razorwing or Fiend/Razorwing seems to be the way to go, since these give you good ways to take care of multiple units and also suck up wounds and allow the Razorwings to do the damage. Note the unit itself doesn't benefit from power from pain, so your base survivability is what you are stuck with and pain tokens may be 'wasted' on these guys unless you have ICs around which can move those tokens. A big negative against ICs and Beastmaster units however is they lose their ability to charge 12". Be very careful with this or you may be unable to assault at a very important juncture.
It's also important to note that Beastmasters only have Ld8. This is massively better than the Ld5 of the beasts but still pretty poor for a very scary combat unit. This isn't so much an issue in combat as you'll quite often win but against tank shocks you have a 2/9 chance of failing. Not great if you've got multiple units against multiple tanks.
All in all however Beastmasters are a great combat unit. They are fast and have a bunch of attacks to scare any opponent with and combining the different types of Beasts can ensure a unit is decently durable against different types of firepower whilst still combat effective.
Conclusions
All three of these units are really good and fill a very particular role in the Dark Eldar army. Scourges are mobile fire support, Reavers are mobile close-range support and Beastmasters are a great combat unit. This means there are very high opportunity costs for each unit in terms of what you are losing by not taking the other unit. Beastmasters in particular have this as you lose access to heat lances and AP1 which can really help a Dark Eldar army pop tanks. At the same time a single unit of Beastmasters can be great and use either Scourges or Reavers as your 'main' Fast Attack.
