The Eldar Datalslate has arrived as part of GW's advent calendar release schedule.
As per GW's site:
The Eldar Ghost Warriors contains 1 Eldar Wraithknight; 3 boxes of Eldar Wraithguard; 2 Eldar Wraithlords; and 3 Eldar Transfer Sheets which can be used to add Craftworld markings and heraldry.
So we have 15 Wraithguard, a pair of Wraithlords, and a single Wraithknight making up this formation. Unfortunately, the Wraithguard can't take any transports so they will be hoofing it.
The formation does come with some interesting special rules, however. As reported over at Faeit 212:
Desperate Measures: All Friendly units in this unit and those within 6" have Hatred. (re-rolls in the first round of close combat)
Wraithbone Wall: Any friendly unit that receives a cover save from this formation form the intervening models rule, receives a cover save of 4+.
You can definitely do some cover save shenanigans with this but given how common Ignores Cover is in the game these days I don't think it holds much water.
The bubble of Hatred is also kind of meh. But if you were building an assault list, you could do worse than having that many pissed off Wraith-constructs whipping everyone around them into a fit of hysteria. Maybe splash in the Avatar to project Fearless and add a dash of Harlequins. Might be a fun list, but over all not my style at least.
So far, the winner is definitely the Tau Dataslate.