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.
On the Tabletop
In stark contrast to the fast yet frail Craftworlders who've come to rely on them, Wraithguard are a slow and sturdy anvil that can weather firepower that'd lay low their still living kin while delivering rather obscene punishment in turn. In 8th/9th edition, Wraithguard stand strong at T6 and 3W apiece, protected by a 3+ save. Durability of this scale easily rivals the likes of the Custodes or Death Guard, though a maximum range of 12" on their Wraithcannons and a rather plodding 5" movement speed will ensure that they'll need to endure a few volleys before they can get into range to retaliate. To this end, a Wave Serpent or utilizing the Webway Strike Stratagem is necessary to swiftly get them into a position to deal some damage before they get worn down. What kind of damage? Regardless of their loadout, Wraithguard pack around S10 AP-4 weapons; every conventional target in the game is wounded on a 2+ or a 3+ while only units with an armor value of a 2+, invuln save or FNP will even have a chance at shaking off the pain. Equip the anti-infantry D-Scythe to lay down d3 shots per Wraithguard for 1 damage a hit, or the single shot Wraithcannon to slam monsters and vehicles for d6 damage apiece; plenty to pop a tank a turn, assuming your dice rolls aren't cursed. Supporting Farseers and Spiritseers can help guarantee that every hit lands and deals some damage.
The Drawbacks? Wraithguard are expensive. 38 points per model for Wraithcannons or 48 points per model for D-Scythes, these spirit warriors will take up a significant portion of your budget, not factoring in any Wave Serpents needed to ferry them about the field. The Webway Strike Stratagem does alleviate that potential tax, but once deployed, your Wraithguard can quite easily get trapped within enemy territory with no escape options should you miscalculate your positioning. They also face rather steep competition from Fire Dragons and Dark Reapers as hard-hitting anti-vehicle/monster specialists. Fire Dragons are very nearly half the price of a squad of Wraithguard and are substantially easier to cram into a transport while Dark Reapers, though nearly as expensive, can fire upon their foes from substantially farther away and deal a consistent 3 damage per hit, something the rather whimsical d6 damage each Wraithcannon deals can occasionally disappoint at. Wraithguard also chafe against the Dark Reaper's flexibility, as their Reaper Launchers also allow them to fire 2 shots that deal a flat 2 damage each, making them much better at dealing with infantry of all flavors than the D-Scythe, which can also disappoint with it's variable d3 shots and rather flacid 1 damage per wound.
None-the-less, Wraithguard and their Wraithblade relatives are the most durable infantry you have and can become a force to be feared with the right support.
The core issue with the Wraithguard remains the same: price. Eleven power for 5 of them is a bit ahead of the curve, and the all-but-mandatory Wave Serpent almost doubles that cost (which is offset by its upgrades being free). Adding a Spiritseer increases the cost to almost a quarter of a 100-point army. For comparison, Fire Dragons only cost three-fifths of what a unit of Wraithguard costs, although they'll admittedly still need the same Wave Serpent (or the more expensive Falcon) to move around.