Fortresses

Naturally created structures that formed in large lakes of mana, the fortresses draw a guardian to tend the mana and remove any pollutants.
    Fortresses were once the seats of magic power. When magic faded they were abandoned and became ruins attended to by ghosts and the occasional immortal.
     Guardians once drawn to their fortress are disinclined to leave their place of belonging, and become dependent on the magic produced from their pool, eventually becoming consumed by it. When a guardian decides to 'die' they leave their fortress and the mana that sustains their being fades and they die a short time later, especially if they've lived out their race's normal lifespan.
     Because fortresses have large amounts of a specific mana pooled around them they tend to make small rifts into other dimensions with the similar types. The guardian often repair these rifts and keeps the mana in circulation, even if slow circulation; least it become polluted and draw creatures such as demons and their servitors to it.