In the Arrow episode 23 of season 4, "Schism", why was Damien Darhk not able to use his magic at Arrow when he was surrounded by the people of the city and how did Arrow's eyes become yellow while fighting Damien Darhk?
In Arrow S04E20 aka Genesis, a shaman named Fortuna explains that the idol channels darkness and death but it can be countered by opposing force that channels light and hope but during practice Oliver failed because his internal darkness was more.
In the last episode when all the citizens start rioting, it was Oliver who inspired them and gave them hope. This is the hope which he channels to stop Darhk.
Those glowing yellow eyes are the representation of that channeled power.