Greetings. I have killed the marilith and i don't know what to do with the crystal. I cannot go back, there is nowhere else to go and it cannot be hit by anything. I got the spellbook and the 2-handed sword, but unfortunately nobody in my party can use it - in case it is supposed to be used for destroying the crystal, although i have only non-evil characters and a barbarian among them. Did i miss something? The only thing i can think of is the room with the 2 frozen dryads, who i didn't find any way to revive.

I managed to destroy it with a fireball. After returning to Athkatla i am presented with the option to kill Tian and keep the spellbook, because my PC thinks that he might unlock its power when he will be a stronger wizard. Is it a hoax, or will something happen if i keep it?

Crystal can be destroyed only with fire, but fire is meant as a spell with area fire effect. No fire weapon, only area fire based spell. If you have a mage in your group, try to use some kind of area fire based spell and you´ll see everything will continue seamlessly...

And I bet you can found a wand of fire somewhere in the tower. But now it´s too late to come back...

EDIT: ...already answered by yourself... About the book - in V2 it is just a cosmetic option. Wait for V3, it will have some meaning in later stage of game.

All right!