Penetration and cast level. Also, blood vengeance.

[Meglobob]

Baalz said:
FWIW, if you have more than 2 natural eyes you can use multiple magic eyes without going blind. The first magic eye gives you the affliction "lost a couple of eyes" which suggests you'll lose the rest of them by using another eye, but in fact the affliction doesn't change with a second eye (only had to two slots, so not sure if you can replace all your eyes)

Also if you give 2 eyes of the void to a fountain for example you get 2 affictions with the description, 'lost some unimportant body part, no effect', so fountains and suchlike can never be blind. But AFAIK they get +4 penetration bonus still.