At the beginning of your upkeep, each opponent chooses 'purity' or 'deviance'.
At the beginning of your end step, if you cast a non-green spell, put a +1/+1 counter on each creature you control for each opponent that chose 'purity'. If you cast no non-green spells, put a +1/+1 counter on each creature you control for each opponent that chose 'deviance'.
See Challenge # 115.
Pleasingly riddly.