As I've just reread xenology i was thinking of some way to include all the races mentioned therein. However the Umbra don't fit the Profile for a fleet, but could be cool as reactive terrain in Deep space!
firstly I'll give a fluff summary!
Umbra are little black spheres that float through our universe existing mostly in the warp having extra dimensional properties (existing in the warp and real space at the same time!).
They are extremely dangerous being able to manifest 'shadow limbs' to defend and attack (the pictures make them look really scary!) Shadow limbs can be made out of shadow like oil turning soild (even from the pupils of eyes!!).
They are attracted to warp engines and all warp-imbued elements of vessels. (They might be attracted to Warp Rifts...In which case they'd reappear on the board in a random location!)
Almost indestructable, only the most powerful weapons can destroy them! However they are adverse to bright lights, and if completely covered by strong light they will die! (this is pretty hard to do though!!!)
It is indicated that the Umbra were once an Old One or 'God' (he who lingers) who was chopped up into little pieces by Slaanesh! (I suppose this could also be refering to Khaine...)
It is theorised that Umbra 'swim' around exits to the webway (mostly in space, but occasionally on a planet!)
Umbra can potentially overwhelm an entire ship! But usually just casue a bit of damage and then are driven away.
So I was thinking somthing along the lines of Umbra Shoals randomly placed on the board. They could move like orbital mines and cause a Critical hit (or just a H&R) when in contact with a ship. (There is no defence, apart from possibly armour!!)
They should ignore, ordnance, terrain and ships that don't have warpdrives (and possibly Eldar! It would help balance my EHR in Deep Space for them!
)Cheers,
RayB