Wraithguard

They also need no air to breathe - something GW tend to forget.

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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, mighty Warp weapons too heavy for normal Eldar to move. 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 of foot"), and as they "see" the world with only their psychic senses, they are often slow to react to the battlefield (each turn, a unit of Wraithguard not within range of a psyker has a one-in-six chance of doing nothing).