Kari Zev does not have summoning sickness and cannot be blocked and killed by an opponent.
Steps
At the beginning of your combat phase, Helm of the Host triggers, creating a nonlegendary token copy of Éomer with haste.
Declare Kari Zev as an attacker.
Kari Zev triggers, creating a legendary 2/1 Ragavan creature token that is tapped and attacking.
Holding priority, activate Thermopod's second ability by sacrificing Ragavan, adding ({R} magic symbol) .
When Ragavan dies, each Éomer that hasn't triggered this turn triggers, untapping all creatures you control and causing you to get at least one additional combat phase after this one.