Check the specialist games website to see if the rles are still there.
Basically it is a chaos pseudo-submarine/pseudo-poo vessel.
It does not start on the table as usual. You nominate a point 15 cm away from a capitalship (yours or your opponent's) and you place the mode there in a "transition mode". It cannot affect anything on the table or be affected by anything on the table himself while in this situation. Its only rule in this case is that it causes a -1 leadership to all enemy vessels around him in 15 cm or so.
While in this state it CANNOT MOVE: this is the poo thing. At the end of your turn, in the ordi phase you check if it fully materializes. If it does from that point onwards is a normal vessel. Can't remember if you still cause the -1 leadership to the enemy or not when fully materialized.
At the start of any of your tirns you can decide to make him "phase out" again, so that he returns to etheral mode (to the warp). Then it can return back and depending on how your roll in a nifty table and how many turns it spend on the warp he can get repaired hit points and the like. If you do this even once it counts that you have disengaged it for VP, even if he returned to the table later in the game.
The upgrade is expensive, BFG games are not long enough to making the engage7disengage thing worthy and it being unable to move while etheral turns it into a sitting duck waiting to be at least rammed when he appears since the enemy will have had plenty of time to position itself or flee the area so it is badly situated for any further action when it appears.
poo explanation, but you get the basic layout of the rules and my impression over them. At first I thought it could move while etheral, but once you see it can't....
Still, makes for a nice model if you convert it well
Regards,
Xavi
I guess Brozza (spelling?) wins hands down there, though