Where in the underworld is Donn?

I know Ive meet him and I thought I marked the location but now I can't find him to give him his puzzle ball. The pirate sent me to the land of endless time but I don't see the Death God there...

Pirate Dedman tells you, "You can find him in his lair. The entrance is an old, hollow tree in Enclave of Wizards. "

Thanks. That was one if the dialogue bits that went by too fast and/or I forgot.

One tree is Spores workshop and the other has something "huge and winged" and I can't enter it. Am I supposed to be able to get in there?

You already got the Whisk-Away-Wand from there, so no. You can't do anything further there.