As they drift down the Falls, arcana checks will show the small gates upening and closing around them. These gates lead to the plane of Fire.
Not a problem, none of them are big enough to swallow up the boat….
Except that one.
A living maw, this particular gate will pursue the boat in hopes of fulfillment. The boat and all aboard go to the plane of Fire.
Fire elementals, look up: They seem similar in shape to huge dogs, with bright glowing burning centers. They of course attack immediately. (Fire elemental stats?) Their destruction means that a larger elemental comes over. Hopefully someone speaks Primordial, because this farmer is mad! He will appear very threatening, but (Insight 30) is actually somewhat frightened by the solids in his land. If attempts are made at communication, he will seek to send them elsewhere. Diplomacy 30 will have him create a firestone, which when swallowed allows conversation with anything that has “Fire” as a subtype.
He had always heard that the mixed planes were terrifying places with monsters running chaotically rampant. Faced with peace, he will offer directions, but cannot help them get back to regular reality.
Arcana check of 30+ lets someone still see the gate-Maw, 35 feet up in the air, rising. Insight 35+ lets you see it’s smug about ingesting the party, and is trying to move along, but is tethered psychically to the boat. This “tether” is slowly fraying, giving the party maybe a day or two before it will be freed. The Maw will not eat anything from this side of the gate. For help, the farmer will direct them to the city. If they’ve done well by him, he will give them a ride and a name: Peerhodeemus, chancellor to the king of this domain.
If party has to deal with it themselves, it is a skill challenge, using up time. It will be very hard.
The party’s unique nature here gets them an entrance into the Inferno, where they find that all resistances to fire are removed. However, if they are behaving, none of the guards have more than curiousity about them. A couple will offer to trade items (blazing armor, fiery weapons, consumables: fire bombs (an elemental snack?), reagents that add fire damage. Rituals can be found if decent diplomacy checks are made.
Peerhodemus explains to the group that the Maw, like others of its ilk, dump things out of one plane onto another, trying to satisfy their need for chaos. Sadly, Peerhodemus cannot help the group directly, but…
A huge water elemental has recently invaded. It is possible to trick the maw by presenting it with the elemental, thus taking it, and our party, back to their plane. The elemental is of course creating all sorts of havoc, but has been contained—just not destroyed. If they take care of it….?
(Insight: Peerhodemus is hiding [History] that killing it offends the Waterkin, and they would wage war for the crime). He offers a bottle that can contain the body of the behemoth.
PCs are led down into the bowels of Inferno, to find the giant water elemental, do battle, and collect its body for the Maw. The bottle is now an endless bottle of water.
The maw drops greedily down, siphoning up the corpse, boat, and hopefully the PC’s, shooting them back into the regular plane.
A final reward for survival through the plane of Fire: the fire resistance that any given member may have is now removed from their item, and internalized.