Retribution/Wraithguard/Strategy
Wraithguard are very powerful units and should not be taken lightly. They have one of the most powerful, Tier 2 ranged weapons in the game, and ignore suppression. However when not used properly, they will most likely kill your own units.
The most important upgrade is the Warlock, and should always be bought unless you plan on keeping your Wraithguard around a spiritseer (Farseer or Warlock - the Warlock from a GU squad works well also). Otherwise Wraithguard are terribly slow and almost ineffective. Note that when the unit that was providing Wraithsight to the Wraithguard dies, they become stunned for a brief time. Any Wraithguard under the effect of Wraithsight will have blue swirling arua around them.
The most powerfult weapon in the Wraithguard arsenal is the Wraithcannon. It is effective against all unit types, suppresses infantry, and does splash damage. A volley of three Wraithcannons have enough damage to completely wipe a Chaos Heretic Squad, and several other low health squads. Though the Wraithcannon is very powerful, it is very slow to fire. It is ideal against stationary foes, making it useful setup units and units in buildings/behind cover, as they less prone to leave or get away. However most units on the run can easily outrun a volley without being harmed. If an enemy melee unit is charging a Wraithguard unit (and they should be, but more on that later), do not expect the wraithcannon to slow them down and supress them, as they will often miss. Since the Wriathcannon can be fired at the ground, a counter to running opponents is to lead a them and fire to ground hoping they will run into the volley; thereby supressing them and making them an easier target to hit with the next volley.
Wraithguard have very high health and are considered heavy infantry, so they can take a lot of small arms fire. They are ideal for drawing suppressive fire and taking out the suppressing unit. However their greatest counter is melee. Wraithguard have no melee resistance and virtually do no damage in melee, so keeping melee off of them is they key to their success. A shuriken cannon is very helpful for keeping melee units from charging and if they do the supression slows them down enough to where the Wraithcannon and tear them apart. If you face an army with Wraithguard, your top priority is to tie up the Wraithguard with melee and force them to retreat. If you do not have any melee units, force a ranged unit to melee the Wraithguard (the Wraithguard do virtually no damage in melee). If you leave units stading still for too long and have no melee against Wraithguard, you will surely lose units.
Since the Wraithcannon is splash damage, friendly fire is always an issue. You want to avoid having reletively low health melee units (such as banshees) attacking the same unit as the Wraithguard; you will lose Banshees. Also the same rule applies to Wraithlords in melee, as they will do more damage to your walker than your opponent.
As a side note, when talking over a VOIP program (Ventrilo, In game VOIP, Mumble), one can easily confuse "Wraithguard" for "Wraithlord" as they are very similar names. If this problem arrises, try to call the Wraithguard something else entirely. We (the authors of this article) use the term "ostriches" - unorthodox, but effective.