In the Simarillion it says that many Maiar were drawn to Melkor in his splendour and others he corrupted with treacherous gifts. What were these gifts?
Melkor at the beginning doesn't have any particular provenance, as is noted in the Ainulindale:
Following his rebellion in the Music he became associated with elemental extremes:
However, the Valaquenta notes that he then wasted his power on domination and tyranny:
The closest indication of what the gifts Melkor gave to his followers is found in the conversation between Sauron and Ar-Pharazon in the Akallabeth, but there is (of course) nothing to say that these were the only such gifts, or even that it was ever anything but a lie:
I'm not aware of anything else to indicate what the gifts may have been.