Coeus, Spring of Knowledge

Whenever an opponent casts a historic spell, they must pay {x} more mana, where X is the number
of historic spells you have on your battlefield, if your opponent cannot pay the mana cost, or chooses not to, put a knowledge counter
on the historic spell. at the beginning of your upkeep all historic spells with knowledge counters come under your control.
Much Knowledge has been lost... I intend to keep it.