Invaders of the Swamp

Each time your opponent casts a creature spell, as
long as you control less than three invader tokens, create a tapped 1/1 red and green invader token
with "Desperation. When this creature despairs it
gains +2/+0 until end of turn. This creature must despair when it attacks."
Neither player can cast creature spells as long as
Each player controls at least three creatures.