This is "Wizard of the Grove" by Tanya Huff.
This review on Amazon matches a lot of your description;
A lengthy prequel pits Rael, son of the hamadryad against the evil
wizard, Kraydak. Rael survives to beget a line of kings who are part
Elder Race. Eventually Crystal, the true heroine of this duology is
born. Kraydak, the wizard also survives. He has been waiting through
many boring thousands of years for the birth of Crystal, the only
human who might give him a magical run for his continuing existence.
Meanwhile he amuses himself by building towers with human blood as
mortar, torturing various walk-ons, etc.--even in this early novel,
Tanya Huff writes zingy, attention-getting prose about evil--you gotta
hate her bad guys, even though they're often her most interesting
characters. Presumably the good guys should be interesting, too. My
only problem with Crystal is that she doesn't have room to grow. She
starts out as a total knock-out with emerald eyes and silver hair, is
born into the royal family, and is the world's most powerful wizard,
except for Kraydak.
Oh, and one of the characters is apparently named Lord Death.