Normally, Eldar are very keen on leaving the spirits of the dead to rest in peace, as they have earned their respite from the grind of battle and are finally safe from Slaanesh's gaze, but some battles require more soldiers than a craftworld's living population can sustain. When this happens, specialized seers called Spiritseers reach into the craftworld's Infinity Circuit and guide a soul back into a spirit stone, which is then placed into a robotic wraithbone body.
Wraithguard are much stronger and tougher than the average Eldar, and even most Space Marines. They carry wraithcannons or D-scythes, mighty weapons that blow holes in the fabric of realspace and tear enemies into the Warp itself. These weapons are too heavy for normal Eldar to move around except on support platforms. Unfortunately, being yanked out of the afterlife makes it difficult for the spirit inside a Wraithguard to function; they aren't lively or quick like their living counterparts (they are one of the few Eldar foot soldier units without the Fleet rule), and as they "see" the world with only their psychic senses, they are often slow to react to the battlefield. It used to be that each turn, a unit of Wraithguard not within range of a psyker had a one-in-six chance of doing nothing, but that rule was removed in 6th edition. In Dawn of War II This was represented by debuffing the Wraithguard heavily if the player takes out their attendant Seer.
In an utterly baffling move by GW, both weapons equippable by Wraithguard have now become destroyer strength. Yeah, try charging a unit that gets D3 autohits on your assaulters at strength D. This is especially concerning, as the fluff for D-scythes is that they cut people off from their souls. We're not sure exactly how cutting souls out from peoples bodies somehow equates to VAPORIZING TANKS. Than again, it could actually makes perfect sense: D-weapons never explode vehicles, just leave wrecks as they strip away HPs, so scythes do not evaporate or even scratch the vehicles themselves—they only kill everyone inside. Though, this hardly matters any more with 8th edition, bringing these stupid things to a reasonable level instead of autokilling everything that ever existed with impunity.