I haven't quite figured out all the details yet, but I'm thinking that it would be a sort of telepathic communication. I'm not quite sure where you'd learn a dead tongue unless they decided to hit up some other cities and gather supplies before they went and found a language buff in the process.

There'd be other things to do during the campaign, this was just the outline of the main plot. For a DM wanting to take this idea and run with it, they can fill in the rest with whatever they want, but if I had to keep expanding on the idea I'd say that most of the subplots would also be centered around what problems killing the other three defenders has caused, such as odd mutated creatures cropping up out of nowhere, occasional flashes of weird weather, etc. Just odd things that nobody thinks are connected.

In terms of letting him live or killing him, there'd probably be other routes to go instead of just leaving him there. They could bring back the leaders of one or more of the main races to talk to Merrith about it, but being the last I think he'd be substantially weaker than the other three due to the strain put on him by having to hold the king back by himself. That should keep it a bit more balanced and make it easier for the party instead of insanely hard if they do decide to take him out.

The politics angle would be interesting, such as having one of the PC's be a diplomat from their race, and the cults could serve as side quests to collect things for or kill for a bounty, but I'm not sure that entire towns or cities would be pro-dragon or against it. Really, everything's just here to be played around with.

The final fight against the king if they decide to kill Merrith wouldn't be too much harder than his own fight, due to being pretty weak when he got locked up. His deal with the demons probably went somewhere along those lines, which could also give the party some allies during the fight if the DM thinks they're too undergeared, otherwise the demons could just chill on the sidelines and wait for him to die so they can collect his soul and whatnot. And while he's been locked up the king has had absolutely no idea of what's been going on in the world, so there's a dramatic speech about him being back to reclaim his empire right before he looks around and realizes there isn't an empire anymore.

I've got no idea about the party's problems if they split, though I imagine it could make another set of sidequests for the ones who want to kill Merrith, gathering allies and getting more support. As for the ones that don't want to kill him, they'd try to do the same, just finding people that want to defend him. They could even try to race against each other to see who can gather more support quicker.

Thanks for taking the time to look at the idea.

Its great but I wanna try something else for once

yeah the DM straghit up lowerd the amount of Monodornes, and lowered the Barbed Demons health from 100 to 40 becuase our rolls were just so bloody appaling (2 nat 1's, though actually Gordo did roll a nat 20)