Kel Asai, Athlastur Prison

{t}: Add {c} to your mana pool.
{w}{u}{b}, {t}: Exile target creature until you lose control of Kel Asai, Athlastur Prison.
At the beginning of your upkeep, pay {g} for each creature exiled this way or sacrifice Kel Asai.
Captives aren't bound or chained. The roots themselves are an endless labyrinth; they'd never escape anyway.