The Mists of Is is a giant all-powerful galaxy/cosmic mist that was the first thing to exist, according to Frostfallian mythology. They house and created the Cosmos Mountain, the gods and dragonlords, Mrethus and mark the borders of the Places We Know. They are partially intelligent, and are sometimes depicted as Fate itself.
The Mists of Is were once said to be a goddess, known as Is (pronounced eyes). She then sacrificed herself to create the partially sentient Mists of Is, which then created three things - the Elder Gods, the Dragonlords, and the Cosmos Mountain. After the gods and dragonlords warred for millenia, the dragons were defeated and the gods sacrificed themselves to create the Seven Realms, the Mists of Is created new entities that ruled as many different beings according to the beliefs of the mortal races. It is said that when the End of Days comes, the Mists of Is shall destroy all and begin life anew in the cosmic planes.