I was going to do the tomb of the beguiler after a while of putting it off. (I was all the way to Mooshu before deciding to do it.) Then when I went to do it, it wasnt in my quest log anymore. If you take too long to do a quest, is it taken out of your spellbook?
No quests are not normally deleted unless a special event has passed or it was deleted due to a major bug. I held this quest off for too long too and i lost it.I think this quest is bugged
Quest you receive when you first enter a Dungeon (areas that require you enter via the Sigils and the countdown timer) are only active while you are inside that Dungeon. If you leave and the Dungeon is reset, those quests will no longer appear in your Quest Journal.
Once you reenter that dungeon you will receive the entry quest again.
The Vault of Ice is a good example of a Dungeon that has quests that are only active while you are inside the Dungeon.