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 Altar of Dementia by sacrificing Ragavan.
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.
Resolve the Altar of Dementia ability, causing target player to mill cards equal to Ragavan's power.