Shadow of Mana

Whenever you play a colored spell, put a +1/+1 counter on Shadow of Mana.
At the beginning of each end step, if there are five or more +1/+1 counters on Shadow of Mana, sacrifice it and deal damege to target creature or player equal to the number of +1/+1 counters on Shadow of Mana.
It feeds on power, yet cannot hold much.