Infinite creature LTBInfinite creature ETBInfinite death triggersInfinite combat phasesInfinite untap of creatures you controlInfinite mana creatures you control can produce
How it works
Declare Mardu Siegebreaker as an attacker, triggering its second ability, creating at least two token copies of Éomer that is tapped and attacking.
Put all but one Éomer token into your graveyard due to the legend rule.
When the Éomer token(s) die, you will get at least one Éomer trigger, causing you to untap all creatures you control and to get an additional combat phase after this one.