Erebos's Influence

Born from Heliod's shadow and then banished to the Underworld, Erebos claimed dominion over that desolate realm. He sees it as his duty and his right to ensure that those who enter his realm remain there for all time. Within the Underworld, only the relative paradise of Ilysia is sheltered from his influence. In the other four realms of the dead, his will is law.
Mortals typically fear death, yet death is the inevitable end of each one's time in the mortal world. Erebos, having long ago come to terms with his own banishment, teaches his followers to accept the inevitability of death and the often unavoidable tragedies of life.
Although Erebos forbids souls from leaving the Underworld, some escape his clutches. Generally such souls are beneath his notice, but he sometimes sends agents to retrieve those he has particular interest in.

Erebos's Goals

Erebos wants above all to maintain his position of absolute authority over the realm of the dead. He lashes out at anyone who aids souls in escaping from him or otherwise tries to subvert the inevitability of death.
Erebos is patient, usually willing to wait for souls to come to him, but he does interfere in the mortal realm on occasion. When the other gods displease him, he sends mortal agents or Nyxborn monsters to kill the worshipers of the offending gods and spread despair.
Many of Erebos's schemes target those who escape his clutches. He regularly sends agents to retrieve the Returned soul or the wayward eidolon of a hero of great renown who has succeeded in returning to Theros. If any of the dead, renowned or otherwise, escape the Underworld fully intact—without identities erased—Erebos will spare nothing to make an example of the fugitive and any who provided assistance.

Divine Relationships

Erebos loathes Heliod, who banished him to the Underworld eons ago, forever severing him from the world of life and light. Erebos takes particular delight in profaning Heliod's temples, defiling the raiment of his priests, slaughtering his worshipers, and claiming the souls of his champions. Erebos believes that Heliod will one day challenge him for control of the Underworld. He looks forward to that day, certain that his lesser brother can't possibly prevail within Erebos's own territory.
Erebos holds a special grudge against Phenax. Phenax readily teaches others how to escape the Underworld as the Returned and even sneaks into the Underworld to assist them, earning him Erebos's undying ire.
Pharika and Karametra understand, better than many of the other gods, that life and death are closely intertwined. Pharika's tinctures can either extend a life or snuff it out, and Karametra's planters know that reaping follows sowing in turn. Although he appreciates what these gods have in common with him, Erebos occasionally feels the need to remind them that the dead are his, and other gods' acceptance of death's inevitability doesn't make them masters of it.
Erebos has no true allies but operates closely with Athreos and Klothys. Athreos is one of the few beings Erebos trusts to be as committed as he is to ensuring that the passage into death is a one-way trip. Klothys, too, would see the dead remain dead, concerned as she is with the acceptance of destiny.