Kaarth, Hellthunder King

Kaarth costs {b} less to cast for each creature that
died this turn.
Kaarth costs {r} less to cast for each creature you control that dealt combat damage to an opponent
this turn.
Haste, Menace, Trample
When Kaarth enters the battlefield, other creatures
you control gain haste. Untap those creatures.
After this main phase, there is an additional
combat phase followed by an additional main phase.