FormerlyHemlock
Hero
A secret weapon that can destroy Hell if [Asmodeus] is overthrown is a good plan. Well crafted rumors of the secret weapon work almost as well as the real thing (maybe better, it is hard to destroy a weapon that doesn't exist).
Also, rumors are a necessary component to even a real doomsday plan. A doomsday weapon is no deterrent unless people know you've got it.
An interesting question: what would a hypothetical doomsday weapon do that the inhabitants of Asmodeus' domain would hate and fear more than the status quo?