Return of Reckoning

Race Dark Elf, the task was completed up to the 3rd point, killing the boss, during the battle with the boss he disappeared.
The task was considered as failed. It was decided to cancel the task, the task cannot be taken again from the item.
The first part is completed and Book of Boom I is in the inventory, the second part cannot be taken again.

You should be able to retake the quest from the item that spawns the boss (he probably timed out from killing slowly). I had the same thing on an Empire char and after a relog (it was actually a few days) I was able to retake the quest. So give it another try

I think when they first released the quest if the boss disappeared you were screwed. But you should be able to drop the quest, and then right click the item or talk to the npc again and it will reset.

I barely solo'd the empire one with a rather twinked lvl25 or lvl26 knight, you'll probably need help on most classes.

for the book of boom II -you may abondon the quest and resummon via the desk there in high elf camp