Wild Mutation

Players can no longer cast spells or play lands. At the
beginning of your upkeep, flip a coin for each creature.
If heads, it gets a +1/+1 counter. Else, a -1/-1 counter.
After that, put a survival counter on Wild Mutations.
Then, all creatures get -2/-2 for each counter on Wild
Mutations. THEN, for each creature with the same
power/toughness as another, create an X/X Offspring
creature token, where X is the Power+Toughness of
the creatures. All creatures gain "Must attack at combat."