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
Wraithguard have only two options, making them easy to put together for your army. They are either 40 points with Wraithcannons or 45 with D-scythes a pop. This means that the smallest possible unit of 5 costs either 200 or 225, with no mixing of weapons allowed. With the max unit size these costs double, but you should not go above five models per unit.
Wraithguard are tough: at T6, W3 and a 3+ save it requires concentrated fire to take them down. Paired with WS3+, S5 and fists that have AP-1 and D3 damage, you'll end up with a unit that might surprise you in combat, but don't count on them as frontline fighters. Their main attraction is their guns: both of whom being S10 AP-4 monstrosities that can rip all but the toughest tanks apart in one barrage. Their traditional weapon is the Wraithcannon, which is a 12" Assault 1 weapon with D6 damage. Not the most reliable weapon against units of infantry, but a single volley can really ruin the day of a tank or monstrous creature. Make sure to fire them one by one if other suitable targets are within range: it'd be a shame if you end up needing only 3 of the Wraithcannons and waste the other two shots with overkill. The other is the D-scythe, an Assault D3 D1 weapon more suitable for blowing up TEQs and if you're lucky blocks of infantry. While it might seem a downside that you don't get to roll for the weapons in turn, in effect this means that you have a two in three chance to land either the average or the best possible result that you'd otherwise have less than half of a percent chance of rolling. On top of that they hit automatically so you'll keep a consistent damage output when dealing with charges. Finally, thanks to their Implacable rule it's not a disaster in case they get charged: they can fall back and still shoot in the same turn.
Having a Spiritseer nearby greatly benefits the Wraithguard. While they fortunately no longer crap themselves the moment there isn't one nearby, a Spiritseer grants rerolls of 1 on to-hit rolls to units within 6" of the Spiritseer. Given the VERY short range of Wraithguard weapons, this is easy to pull off. As for powers, the Conceal/Reveal power works best of them: putting a -1 To Hit on all opponents shooting your Wraithguard is going to be very useful to not die. However, the 66 points that come with a Spiritseer will make the unit even more expensive.
The downside to fielding Wraithguard is that they're slow. At 5" speed they are one of the slowest units in the army, so they'll need a transport. The Wave Serpent is an excellent tank on its own, and it can carry either six Wraithguard or five as the bodyguard of a character. This greatly increases the cost of the unit to properly field it. And it's not like you can drop the Wraithguard off at one point and go on your merry way: Wraithguard are likely to destroy whatever big thing they shoot at but after that they're sitting ducks who need to be moved around to survive. A Wave Serpent can easily double the cost of a unit, meaning you're pouring 500+ points into a five-model unit. This is also their weak point: five Fire Dragons can wreck enemy tanks at almost half the cost and can be moved around easier. While they lack the durability of the Wraithguard, the lower cost more than makes up to this. And to match the point cost you'll have to field EIGHT Fire Dragons, who have a lot more firepower than five Wraithguard when facing a tank. They'll wound less often with S8 weapons, but they either get rerolls or get to pick the better number on the damage roll based on what they're using.
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.