Sienna the Unchained

When you cast a red spell, put a +1/+1 counter on Sienna the Unchained. When Sienna has 5 or more +1/+1 counters,
remove all +1/+1 counters from Sienna and deal 5 damage to Sienna and each opponent.
At the beginning of your upkeep remove a +1/+1 counter from Sienna the Unchained.