The Leviathan Pact is generally made by those sailing in Northern waters who meet a large tentacled monster. It typically will destroy an entire boat, killing the entire crew but one. The one who is spared is then branded on the chest and teleported to shore. From there the tentacled beast tells the victim they have been marked and they will be harvested later. The people afflicted by this tend to either go mad or try to gain enough power to avoid this “harvest” for their own safety.
RULES NOTE The Leviathan Pact is actually the Star Pact described in the PHB 1 and thus is just a refluff.