The Great Henge

The first step of casting a spell is to move it to the stack. If this causes the greatest power among creatures you control to change, that new power will be used to determine the cost reduction.

Once you announce that you're casting a spell, no player may take actions until the spell has been paid for. Notably, opponents can't try to change by how much The Great Henge's cost is reduced.

Once The Great Henge's last ability has triggered, you'll draw a card even if you can't put a +1/+1 counter on the creature for some reason (most likely because it has left the battlefield).