Mlandoth and Mril Thorion, in the Cthulhu Mythos, are fictional characters created by Walter C. DeBill Jr., but suggested years earlier by Clark Ashton Smith.
According to the cycle surrounding said beings, these creatures are a sort of cosmic Yin and Yang, whose meeting resulted in the creation of all things (although the terrible Azathoth is usually attributed to this).
But whether Mlandoth is a place, or a conscious entity, or an inconceivable maelstrom of unknown forces and properties outside the perceptible cosmos is not known surely.