"The Necromancer"

[mhagain]

Thû is referred to as a necromancer as far back as the Lay of Leithian:

Men called him Thû, and as a god
in after days beneath his rod
bewildered bowed to him, and made
his ghastly temples in the shade.
Not yet by men enthralled adored,
now was he Morgoth's mightiest lord,
Master of Wolves, whose shivering howl
for ever echoed in the hills, and foul
enchantments and dark sigaldry
did weave and wield. In glamoury
that necromancer held his hosts
of phantoms and of wandering ghosts...

This is actually the passage in which he emerges almost fully-formed; the only things really missing were the Rings and Númenor (the latter of these also came from pre-LotR texts), and of course all the Third Age stuff, but otherwise we have the concept of his survival beyond the First Age ("in after days"), Lord of Werewolves, Morgoth's second-in-command, his liking for temples, his later worship by Men, etc.

So even then, in a pre-Hobbit work, the concept of Thû as a necromancer who meddled with phantoms and ghosts, was all present and correct.