Mana Pit

Mana Pits emanate blue light emanates from a mystical sacarfical pit. These pits are used to sacrifice your minions in exchange for Mana.

Overlord

 * Mellow Hills
 * Golden Hills

Overlord: Raising Hell

 * Mellow Hills Abyss
 * Heaven's Peak Abyss
 * Evernight Abyss
 * Golden Abyss
 * Infernal Abyss

Triva

 * They are not seen in Overlord II, due to the fact you could sacarfice them on the spot(if nessessary), and if you aproached a netherworld gate you'd automaticly refill.
 * The symbol on them resembles the Zora's Sapphire from Legend of Zelda: Ocarina of Time as well as the "Air" symbol from the Legacy of Kain series (from Soul Reaver 2 onwards).