Aggravated Assault and Shriekwood Devourer on the battlefield.
You control up to five lands that can tap to produce at least ({3} magic symbol) ({R} magic symbol) ({R} magic symbol) .
You control a creature with power 5 or greater that does not have summoning sickness, and that cannot be blocked and killed by an opponent.
Steps
Declare any creature with power 5 or greater that cannot be blocked and killed by an opponent as an attacker.
Shriekwood Devourer triggers, untapping at least five lands.
Move to your postcombat main phase.
Activate up to five lands by tapping them, adding at least ({3} magic symbol) ({R} magic symbol) ({R} magic symbol) .
Activate Aggravated Assault by paying ({3} magic symbol) ({R} magic symbol) ({R} magic symbol) , untapping all creatures you control and causing you to get an additional combat and main phase after this one.