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:
To Melkor among the Ainur had been given the greatest gifts of power and knowledge, and he had a share in all the gifts of his brethren.
Following his rebellion in the Music he became associated with elemental extremes:
And he feigned, even to himself at first, that he desired to go thither and order all things for the good of the Children of Ilúvatar, controlling the turmoils of the heat and the cold that had come to pass through him.
However, the Valaquenta notes that he then wasted his power on domination and tyranny:
In the powers and knowledge of all the other Valar he had part, but he turned them to evil purposes, and squandered his strength in violence 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:
For Darkness alone is worshipful, and the Lord thereof may yet make other worlds to be gifts to those that serve him, so that the increase of their power shall find no end.
I'm not aware of anything else to indicate what the gifts may have been.