Goblin Curse

All goblins carry a curse. Once like you, they are now trapped in the prison of their crazed goblin flesh. Only their eyes reveal the truth: a ruined mind watching its body-prison perform terrible deeds.

The curse spreads by attack. Even being attacked by goblins sends the curse, carried on the winds of their hate — hit or miss, it doesn't matter. Anyone attacked by a goblin irrevocably transforms into one in d6 days, unless the attacking goblin is killed before then.

In the Goblin Grinder scenario, Nagel Krat sells the Goblin Cure — which isn't a real cure, only a delay — exploiting this curse for profit.