I'm considering trying out a build for him based on his passives, so sacrificing his spine attack, which has good damage and speed, but is not very mana-efficient.
1. Warp Strike 1
2. Demon's Speed 1
3. save
4. Warp Strike 2
5. Shadow Swap 1 & Demon's Speed 2
6. Elusiveness 1
7. Elusiveness 2
8. Demon's Speed 3
9. Elusiveness 3
10. Shadow Swap 2
11. Warp Strike 3
13. Warp Strike 4
14. save
15. Assassin's Speed & Deadly Warp
16. Shadow Swap 3
17-20. Precision 1-4.
The reasoning behind this is that you can now focus entirely on getting health/armor and attack damage items, and not buy mana items or use up a slot for Boots of Speed or Wand of Speed.
Not sure if this could work, of course it depends on the other team, against a team or like TB, Sedna, and slam Rook, Elusiveness would not be very useless at all (since tb and sedna get almost all their dps from their spells and slam's rook main damage is slam), so you'd drop it.
Demon Assassin is quite a constast to UB, since wtih DA you want all his abilities just as badly (except Warp Area, that appears to be crap and not worth it), but with UB you have so much leisure since the only abilties that are worth it are spit, and ooze(the best ability in the game), and 1 rank of grasp(best stun in the game), only 1 rank of his disease passive (for the aoe in killing creeps), and 1 in his snare, and all of his atk/move speed line.
On a side note, does anyone know if Warp Area removes debuffs, since it makes you invincible, I'm guessing no, but if it did it could definitely be useful against a spit ub or a reg with shrapnel mines.