Twasi, Hippopotamus Prince

Players don't lose unspent mana as steps and phases end and can tap permanents for mana only during their turns.

Twasi, Hippopotamus Prince has indestructible as long as no opponent has ten or more unspent mana.

Whenever Twasi deals combat damage to a player, that player loses all unspent mana and you add the mana lost this way.