Millennium Rod

Regal & Millennium(This scheme costs {1} less for
each time you've casted your commanders this game)
At the beginning of your end step, you may pay {x}. If you do, gain control of target creature an opponent controls with a mana value of X or less.
This ability costs {1} less for each time you've
casted your commanders this game.
(Max one creature at a time.)