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.