Éomer cannot be blocked and killed by an opponent.
You have at least three opponents.
Steps
At the beginning of combat,
Duke Ulder Ravengard triggers, giving
Éomer an additional instance of myriad until end of turn.
Declare
Éomer as an attacker.
Each instance of
Éomer's myriad ability triggers, creating at least two token copies of it tapped and attacking each other opponent.
Put all
Éomer tokens into your graveyard due to the legend rule.
When the
Éomer token(s) die, each
Éomer triggers, untapping all creatures you control and causing you to get at least two additional combat phases after this one.
Repeat from step 2 each combat phase.
Once you have an arbitrarily high number of combat phases, you may have
Duke Ulder Ravengard give myriad to any other creatures you control.
Notes
The combo stops once you have two or fewer opponents. The combo will make progressively more combat phases, but you cannot untap creatures in between using the cards in this combo.
Results
Near-infinite ETB.
Near-infinite LTB.
Near-infinite death triggers.
Near-infinite combat phases.
Near-infinite untap of creatures you control.
Near-infinite mana creatures you control can produce.