Mara is a huge blue demon that doesn't eat monsters unlike the other demons. It is born from a Mara Rune. The rune is created by having no nutrients or mana. That can be achieved by creating a rune with no nutrients. Then heroes have to step on the rune multiple times to drain off all its mana.
Mara can't buff your dungeon like the rest of the demons, but it is a very powerful fighter.