Where is the holy relic in skull castle (the one that the Lich wants)? I suspect it's in the chest in the "lava room", but I can't reach it because the floor collapses and my entire party is burned to death!

You must use Lava walk spell,and then walk to treasure box,and CARRY box to normal floor.If you have not Lava walk spell,you must hurry to treasure box ,and CARRY box.

Thanks for the advice, but I have allready found another way. There is a switch hidden behind some crates to the southwest that makes the bridge stable.

Btw, can i know how to get to this "Lava" place?

The 'lava place' is in the vault. You will need the key from the lich to get in. The vault entrance is that gate underneath the front door.