Dark Lord
The Dark Lord is an unknown villain in the Plague quest series. According to King Lathas, he corrupted King Tyras. However, in Regicide it is revealed that Tyras is not evil. The dark elves are trying to summon him to Gielinor. According to Arianwyn at the end of Regicide, the Dark Lord comes from Zamorak's world. Arianwyn, the leader of the Rebel Elves who lives in Lletya, says: "If the gate to Zamorak's world is opened we all die." It can be said that Dark Beasts also originate from that world, since they come from a "darker dimension", and are found after progressing father through the Plague quest series.
It is hinted by General Hining that the Dark Lord is Zamorak, though according to the Edicts of Guthix no god is able to directly influence Gielinor, or else, a select group of Guthixian followers are sworn to awaken Guthix from his sleep to remake the world as it has never been before. The name Dark Lord is also used in the game to refer to Lord Drakan, though a connection between the two is unlikely.
Iban is also refered to as the Dark Lord by his followers, this seems more likely since it doesn't clash with the Edicts of Guthix.