Thuringwethil, the vampiric messenger of Melkor, is not explicitly named as anyone’s original Maia. While it’s possible she was one of the original Maiar of Melkor, I find the ways other Maiar can be twisted to be so fun to explore
Perhaps she was a messenger originally to Varda, her form little more than the wisps and vapors that frequently make up the Maiar that sere the queen of the stars. She materializes rarely. When she joins Melkor, her form changes and smoke becomes sinewy wings, teeth forming and piercing.
Now, she is a shadow that passes over the stars she once worshiped
Or, Thuringwethil as a primeval hunter, fastened and shaped by Oromë to blur lines, to live in shadows, as an ambush predator who strikes not with great tearing claws but in smaller, burrowing ways. There is nothing unnatural about this and in the natural world there is nothing evil. But corrupted, her predation takes on a new form
Vána’s realm, cycles of life and death, spring and winter and growth and decay could be terrifying even without with the active malice of Morgoth. Thuringwethil starting out as a fruit bat, fluttering in the glades of Vána, a pollinator. Drawn to Melkor for the extremes he embodies. Pollinating the poisoned flora of the Ered Engrin, feasting on flesh as much as fruit.
Thuringwethil originally serving Irmo. Dreams and visions transmitted through teeth and wings. Thuringwethil not only as messenger but a guardian of Angband’s many nightmares