"Need brooks no delay, yet late is better than never."

Éomer, Marshal of Rohan

Haste
Whenever one or more other attacking legendary creatures you control die, untap all creatures you control. After this phase, there is an additional combat phase. This ability triggers only once each turn.

The Lord of the Rings: Tales of Middle-earth

Tales of Middle-earth Promos

If Éomer, Marshal of Rohan and one or more other attacking legendary creatures you control die at the same time (probably due to combat damage), Éomer, Marshal of Rohan's last ability will trigger.

Notably, Éomer, Marshal of Rohan's triggered ability doesn't give you any additional main phases. This means that you will move directly from the end of combat step of one combat phase to the beginning of combat step of the next one.