Where is The Tree of Death?
 

Maybe I'm being stupid but I've wandered everywhere in the deadlands and can't see it

It's in Death Valley, somewhere along the river (I think it's a river).

It's also not far away from the Velisar guy (the one with the black unicorns), and also very close to the beginning of the uphill pathway that leads to castle Necrocom.

These are rough, but you should be able to figure it out, it's really not difficult to find at all.