Maiar

Maiar were Ainur created by Eru and sung with the Valar. They came to the world to build it. They are not as powerful as the Valar. Some of them were subverted by Morgoth, like Sauron, dragons, monsters such as the Balrogs, spirits of flame and shadow.

The Maiar who wear an earthy body are possible to be killed, but they can return in another form.