I go in the Sphere with Valygar ...
I kill everybody and clean everything, i attack Lavok who fails on the ground ... but i can't kill him ... and of course, i can't land with the sphere (i'm prisoner in the sky). I dunno why : i'm sure i tried everything.

Thanks whoever can help me.

ever thought about reading what other characters say to you ? you need to get a demons heart and feed it to the propulsion golem. then you can take the near-dead lavok with you outside of the sphere.