Wraithguard
Wraithguard are wraithbone constructs made by the Eldar to house the souls of fallen warriors.
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 Warp weapons too heavy for normal Eldar to move around on support platforms. Unfortunately, being yanked out of the afterlife makes it difficult for the spirit to function; they aren't lively or quick like their living counterparts (they are one of the few Eldar foot soldier units that are not "fleet"), 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).
Wraithblades
A variant of the Wraithguard that 6E introduced into the sprues are the Wraithblades, a type of more kill-hungry Wraithguard with big helm crests that don't compensate for anything at all. In a way, they behave like Chaos Dreadnoughts, who go insane without someone to cut up. It's just that they get treated a little better by not being ripped out of their shells and getting subject to pointless torture. Instead of guns, they carry around either a pair of Ghost Swords or a Ghost Axe and Force Shield.