Early morn I picked up the new data slate salivating at the prospect of some free USR's for my wraith army. Well, I was not blown away, but after some thought the rules have grown on me. The first thing that struck me was that if I selected the detachment it appeared I would loose three troops choices in the bargain. Unlike the Marine and Tau data slates, the Eldar one is made up of troops. It's a big deal to loose them given the high cost you pay to put wraithguard on the table. Reading the rules more carefully; however, I think if you take a spirit seer in your primary detachment, the wraithguard in the ghost warrior detachment become troops.
First stage in the analysis is to look at the relevent rules:
-If a wraith seer is selected all wraithguard/blades in the ARMY move from elites to troops.
-All squads, vehicles and characters in the army are placed into a FOC slot depending upon their role on the battlefield.
Eldar Data Slate:
-A formation is a special detachment.
-The army list entries for units part of a detachment can be found in the codex corresponding to the faction the formation belongs to, including battlefield roles.
So at first I thought that because an army includes all detachments the spirit seer rule trumps all and wraithguard must be troops no matter what detachment they come from. The problem with this proposition is that the spirit seer rule also explicitly states that the wraithguard shift from elites to troops. The ghost warrior detachment makes no mention of what FOC slot the wraithguard belong to. The problem that arises is that while the first condition of the spirit seer rule is satisfied (all wraithguard in the army) the second condition of the rule (FOC slots) can not be addressed. So I thought to myself rules as written, wraithguard in the ghost warrior detachment cannot become troops.
Not satisfied with this answer I dug a bit deeper. Looking at the data slate rules for these special formations there was a clear link drawn between the data slate and the parent codex. It is explicitly mentioned that individual rules for the units listed as part of the detachment can be found in the parent codex. A number of examples are then listed including battlefield roles. I was not really sure what exactly battlefield roles referred to so I cracked open my Eldar codex to take a look. Under the 'using the army list' heading the rules state that unit entries are placed into FOC slots based on battlefield roles. So there appears to be a direct connection between battlefield roles and FOC slots.
If this is the case then the wraithguard in the ghost warrior detachment now has a FOC slot assigned to it. I believe this proposition is buttressed by the compulsion to look at the parent codex to determine the particular rules of a unit that make up part of the formation. I suggest this means that the wraithguard selected as part of the special detachment has an FOC slot assigned to them. This would then mean that the second component to the spirit seer special rule could be applied to these wraithguard. As the spirit seer rule is army wide and not limited to detachments, the wraithguard from the ghost warrior detachment would move from the elite FOC slot to the Troop FOC slot as soon as a spirit seer was selected from the primary detachment.
I think it is important to note that while the wraithguard in the ghost warrior detachment become troops if a spirit seer is selected in the primary detachment, those same wraithguard do not satisfy the troop requirements of the primary detachment. So 2-6 troop choices must be selected from the primary detachment.
Putting this all together I think the ghost warrior detachment provides a lot of options then when designing an Eldar list. Without taking an allied detachment the Eldar player can (when a spirit seer is taken as an HQ) include up to 9 troop choices and 6 heavy slots (3 of which must be 2 wraithlords and a wraithknight). In essence the Eldar book can now ally with itself to a limited extent without giving up the ability to take a secondary detachment. I think this makes the ghost warrior detachment a very solid choice when building a list.