Larian Studios Forums Divinity - Original Sin Divinity - Original Sin - Enhanced Edition Leandra (Spoilers for end game)

There should be a chance to use the soul forge ritual after Leandra is defeated, as long as you have it and Icara is alive at that point. The dialogue with Zixzax shouldn't matter.
Icara is not scripted to die if Leandra is defeated, whether or not you have or use the ritual.