Among the Valar were the Fëanturi, or Masters of Spirits, who were brothers. Their names were Námo and Irmo, though they were generally referred to by the places where they live, Mandos and Lórien respectively.

Námo (which meant ‘ordainer’ or ‘judge’) was the keeper of the Houses of the Dead. Irmo (which meant ‘master of desire’) was the master of visions and dreams.
Encyclopedia entry originally written by Ainulindale