I've heard every opinion possible on this match-up. Ranging from the extremes of Dark Eldar dominate to Grey Knights dominate with everything in between. This might indicate a perfectly awesome normal distribution of player experiences...ya right. Rather, I think these extreme opinions are based upon players seeing/participating in battles where one general continually exerts themselves. Why? Well personally I believe the two armies are well matched-up but are HUGELY different armies and much like Dark Eldar and Tyranid match-ups, both armies do quite well at hurting each other but at significantly different portions of the game. This is why I feel a lot of people have been led to extreme opinions as whichever army is able to get a stranglehold with their strengths is very likely to obliterate the opposing army and perhaps lead to fewer close games.
Enough conjecture though! Let's actually look at how the armies match-up. Remember, this is the whole codex against the whole codex and whilst common themes in most armies will be discussed, we aren't comparing specific lists or looking to tailor lists against opponents.
Dark Eldar Advantages:
Mobility and range are going to be obvious advantages to any Dark Eldar army - it's why you generally play them after all. Whilst Dark Lances are only 36", many units have the option to move 12" and fire. Against an army with primarily 24" guns, this can lead to early game advantages and more options later game. Whilst Grey Knights in transports can quickly close to that 24" gap, Dark Eldar have the ability to then move away and force the Grey Knights to re-embark and move after them. Whilst there is finite board space, its important for the Dark Eldar army to minimise the amount of 24" shooting the Grey Knight player can get in. This not only means shooting and moving to keep out of that 24" range as much as possible but also to not spread your army out but focus on one flank of the Grey Knight army. This is where their extreme mobility can really come into play and whilst it sacrifices midfield, will allow your army to live longer (shock prows obviously become very important as well to tank shock units off objectives in midfield).
Darklight weaponry is great at shutting down AV11 mech. Whilst Grey Knights have fortitude which can really hurt the Dark Eldar suppression fire, the amount of lances most lists contain (~20 for 1750) means there will be wrecks and weapon destroyed results going through. Since most of the darklight weapons are also spread out across many units, the potential for multiple such results is very high if unlikely.
Poisoned shooting weapons are great against all Marine armies and perhaps none more so than Grey Knights. Grey Knights like to play outside of their mech and to be able to keep up with the Dark Eldar army in the early turns where they don't have board control, Grey Knights will be forced to hop out of their transports if they want to bring their stormbolters and psycannons to bear. This obviously opens them up to massive amounts of poisoned shots which are very efficient against expensive Marines. Whilst it may take four Venom on average to down a small Marine squad, the ability to wipe out whole squads in the shooting phase is hugely important and not always needing to break open tanks to first access the infantry gives the poisoned weaponry a big boost in early turns. Add in that Venoms can move 12" and shoot 36" and they too have the same range and mobility advantage over Grey Knights and remember, they care nothing about cover. If they can see one of your guys, they are going to put some wounds on them quick smart.
Certain combat units of Dark Eldar have a field day against Grey Knights, specifically Beastmaster units with Khymera and Wyches. You have Force Weapons you say? Don't care thanks to invulnerable saves which are better or as good as their normal saves. This means one of the few advantages Grey Knights have in combat is mitigated against very common combat specialists for Dark Eldar and whilst a unit of Purifiers, Paladins or Death Cult Assassins will still hold their own against such units, combat is once again an area the opposing army has the advantage.
Nightshields, whilst uncommon, will be of great benefit against the 24" range weapons of Grey Knights. Whilst less effective against stormbolters compared to rapid firing weapons like boltguns, it can really magnify the movement and range advantage Dark Eldar have in those early turns and particularly help the Dark Eldar isolate and flank the Grey Knight army. Whilst night shields will have little effect against the increased range of units such as Psybacks, LasPlas Razorbacks or Psyfledreads, their real value is in limiting the middle ranged firepower of the Grey Knights.
Pain Tokens will help a lot against Grey Knight's major anti-infantry shooting - stormbolters. Whilst obviously not as effective or widespread as in Blood Angel armies, FNP can greatly blunt the Grey Knights ability to kill lots of T3 infantry quickly. The combination of cover and FNP ensures 3/4 of all wounded infantry will live to fight another day against the main anti-infantry weapon of the Grey Knight arsenal. Add in the usual benefits of extra pain tokens such as being better in combat and fearless (especially against shooting) and smart management of pain tokens can become a huge game changer.
Grey Knight Advantages:
Fortitude is an obvious one here. Dark Eldar lance weaponry is all about causing as much mech damage across an army. Whilst 20+ S8 shots is going to wreck/explode some tanks, the ability to cause shaken, stunned, weapon destroy and immobilised results across the opposing army is where they really shine. Against Grey Knights half of those extra results get ignored more often than not. This really hurts Dark Eldar as Grey Knight mech is very well equipped to deal with Dark Eldar.
Psyflemen are of course at the forefront of this argument. Being AV12 means darklight weaponry is at their most inefficient and with 48" + Fortitude, they are very hard to take down. Dark lances will find it hard to get close enough to shoot them without exposing themselves to a lot of return firepower and even if they do, the Psyfledread still ignores 33% of any results on a successful psychic test. Add in their weapons are S8 so easily pen AV10/11 and each Dreadnought has two actual weapons and they are very hard to deal with for Dark Eldar. Venerable Psyflemen are even worse for Dark Eldar to deal with. Psybacks are also a pain for Dark Eldar to deal with. They have the range to match Dark Lances and whilst AV11 and thus more vulnerable to S8 fire, are numerous enough to make up for this. Their S6 firepower is good enough to hurt AV10 with some degree of reliability and outside of the first couple of turns, will generally be in range. Whilst more vulnerable to firepower and suppression results (weapon destroyed/immobilisation), they are still a right pain for Dark Eldar to deal with.
Whilst pain tokens can nullify the effectiveness of stormbolters, they are still quite a good weapon against Dark Eldar. They have the usual 24" supremacy against regular anti-infantry weapons (i.e. splinter rifles) with the added bonus of being decent enough against AV10 open-topped vehicles. Add in psybolt ammo and they can be very effective against transports though limited by their range, mobility of Dark Eldar and potential nightshields. Whilst they can be relatively ineffective against infantry with pain tokens as well, a vast amount of firepower in the rest of the Grey Knight army is S6 or better. Psybacks, Psyfledreads, Incinerators, Psycannons, etc. all cause instant death against T3 models which can reduce the effectiveness of pain tokens by a severe margin. In combat force weapons obviously ignore this bonus as well though the high initiative and invulnerable saves on certain Dark Eldar combat units means this is often irrelevant.
Grey Knights are a midfield army and against Dark Eldar, there is nothing truer. Whilst Dark Eldar will often have a range and mobility advantage against Grey Knights, by moving into midfield they deny the Dark Eldar some of this. I'll do a post on this shortly but having 24" guns in the centre of the board means you can reach nearly everywhere. The Dark Eldar can still minimise how many guns can reach you for a certain period of time but they cannot do so forever. Furthermore, by sacrificing midfield to Grey Knights, Dark Eldar lose board control and this is very important in objective based missions. By holding the centre of the board Grey Knights have it very easy in terms of moving out and claiming objectives. Dark Eldar will have to force Grey Knights off objectives later in the game and this isn't exactly easy.
Although Dark Eldar will generally have an advantage in combat, particularly units such as Beastmasters or Wyches, as usual, Halberds can throw a spanner in the works. Whilst both of these units still aren't as fussed against such with invulnerable saves and high initiatives themselves (i.e. generally striking simultaneously), Dark Eldar combat often relies upon getting the first punch in to weaken the return punch. Halberds mitigate this and particularly combined with combat grenades means certain units can really close the combat advantage Dark Eldar units have. Add in psychic powers such as Hammerhand and Cleansing Flame to cause extra damage and certain units are better at dealing with Dark Eldar combat units than others.
And this brings us to the final point - lack of psychic defenses for Dark Eldar. This obviously goes hand in hand with Fortitude above but Dark Eldar have no psychic defenses. No, crucbile does not count and whilst is most effective against Grey Knights compared to other armies (no it doesn't affect vehicles), it's not a defensive mechanism and not good enough against all lists to take just to deal with Grey Knights. This means Grey Knights are free to cast whatever powers they want, when they want. They don't have to fear perils and have a very high success rate in terms of casting their powers. Whilst things such as Hammerhand are less useful here since Grey Knights already wound most Dark Eldar on 3's (though upgrading this to 2's isn't a bad thing), being able to cast powers like Cleansing Flame, Fortitude, Sanctuary and Shrouding without consequence is very nice and really lets the Grey Knight army fire on all cylinders.
Conclusion:
Where does this leave us? Both armies have some serious advantages against each other and it may seem like Grey Knights have the edge simply because of Fortitude. Whilst this can really impede upon the effectiveness of Dark Eldar shooting, this factor can be minimised to a certain extent my the range limitation of Grey Knight's midfield weaponry and the exposure of Grey Knight infantry to poisoned weapon fire. Whilst one might assume whoever goes first has the greatest chance of victory, this is perhaps one of the best match-ups to dissuade that. If Grey Knights go first the Dark Eldar player has the most options in terms of deploying as far from them as possible. Whilst the Psyfledreads will very likely be in range no matter what, they can ensure maximum distance against the Grey Knight Psybacks/Psycannons/Strombolters. A further option is to reserve and deny the Grey Knights two whole turns of shooting though the Dark Eldar lose their early mobility/range advantage. If Dark Eldar go first they have a higher chance of knocking out multiple tanks, shooting up Grey Knights and reducing the firepower coming into them for the rest of the game. Grey Knights can of course reserve themselves and do it better than Dark Eldar if they can utilise Psychic Communion though of course the Dark Eldar player still has the initial range/mobility advantage as the Grey Knight army comes on.
What these deployment examples show is that whilst it may seem like one army has the advantage over the other, there are a lot of options and variables which go into it. Dark Eldar need to do their damage as early as possible whilst taking as little damage in return as possible. This means maximising their mobility and range advantage for as long as possible - night shields help a lot here. This means in the early turns Dark Eldar are more likely to have the upper hand but as multiple psycannons get into range, the balance of power shifts to Grey Knights as they have a lot of S6-8 shots which can bring down AV10 open-topped vehicles. This is where Grey Knights need to make their stamp because Dark Eldar will generally have a significant advantage in combat, especially if there are Beastmaster or Wych units around.
All in all it really comes down to which general better applies their army. Hot dice in either army's 'sweet spot' is going to see the game become very one-sided and with Dark Eldar fragility, they are certainly more vulnerable to this than most armies but if you do not maximise your army's strengths, you will lose. Dark Eldar and Grey Knights have very different strengths which come into play against each other at different times. Whichever player maximises their opportunities and minimises their opponents during these times will often find themselves the victor.
