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.
When attempting to enter King Tyras' tent, the guard next to the tent tells the player that 'Zamorak's fury will be released upon us'.