Omicron, the Soul Forge

Whenever a spell is cast, put a unique soul
counter on Omicron, the Soul Forge for each
of that spell's colors. Omicron, the Soul Forge
can only have one soul counter for each color.
You may pay {x} and exert Omicron as it attacks,
where X is the number of counters on Omicron.
When you do, tap target creature. Omicron gets
+X/+X until end of turn.