Balrogs

Balrogs are corrupted Maiar who reside in Angband and serve the dark lord Morgoth, the antagonist of Beren and Lúthien.

Beren and Lúthien (2017)
When Lúthien and Beren enter Angband, she puts the servants of Morgoth under her spell, causing the Orcs and Balrogs to fall asleep.

Appendices
"Balrogs - [In the Lost Tales the Balrogs are conceived as existing 'in hundreds'. They are called 'demons of power'; they wear iron armour, and they have claws of steel and whips of flame]"