There should be a book about the lore of the bet. But where?
A bet long forgotten
The threat is upon you now!
Asking for information the party hears of an old tale of how the Elder made a bet with Lord Khuchar a long dead god, father of the devil army. The deal was thus. If a mortal torn between two worlds would attempt to activate a portal of the Elder a game would start. An island with a scribe and a portal would appear. The portal would summon monsters from the infernal planes in a sequence and the scribe would be the judge of the game. The mortals would either have to destroy the portal thus ending the game for the side of the Elder. Meaning his portals would work. Or the Devils would reach the second Island that is in a secret location and destroy the heartstone there. If they succeed then all portals will open at the Devil’s will and Shekal will fall to the Devils.