The Fallen Star

Indestructible

Equip {3} or equip legendary {0}

Equipped creature gets '{t}: Add three mana of any color" and 'this creature gets +1/+1 for each unspent mana in your mana pool.'

You don't lose unspent mana as steps and phases end.

"She is more precious and more savage than I could have ever imagined." - Petra, Keeper of the Star