They just got to the Temple of Osiris and are completely willing to help the good priests there defeat the evil being that is defiling their sacred waters below the Temple.
I don't find the Underworld optional - one of the key 9 artifacts is located there.
The monster down there has a breath weapon, legendary actions, nearly 300 hp, and has multi attack. It "could" bring them down.
You're right, I forgot one of the artifacts was down there. I was remembering that the PCs could end up down here as sacrifices if the false priests fooled them.
Two things about the Osirium.
First, the Osirium – even with the couple fights that Frog Gods' conversion removed – will probably be more combat-heavy than your usual 1-2 fights per day. It's also implied (at least in the original) that these combats happen with
little to no rest in between... A core element to the Osirium is all the fights and damaging traps are wearing the PCs down. Originally, the time pressure is associated with (a) the river is flowing in one direction, and (b) the threat of the evil false priests. If your PCs have magic allowing them to break that
pacing (which is likely) and/or have dealt with the false priests already, you may want to introduce a reason why they can't rest very much between combats (or at least why they can't long rest between combats). For example – as a ritual reenactment of Ra's solar barge through the underworld journey, the challenges must be faced within one night or else the ritual participants find themselves magically whisked back to the beginning with all monsters restored (this could be narratively explained as a 'regional effect' of the "evil being defiling the sacred waters", if you like).
Second, these combats take place primarily
on a boat while floating on a dark underground river, against swimming (or flying) foes)... that puts the PCs at somewhat of an environmental disadvantage, not just potential of being dragged
underwater (worthwhile to come to agreement as a group about your ruling on "is a PC 'holding their breath' when hit and grappled overboard by a monster or do they immediately begin 'suffocating'") or monsters attacking the boat, but also that they will probably be more
clustered (i.e. on the boat) vs. area effect attacks. I don't think the original or Frog God version mentions this, but playing up that environment could help make for more challenging combats.
EDIT: Ok, third, if the
evil high priest is still around, that's a huge complicating factor when his betrayal manifests in the middle of one of these combat.
Some specific thoughts on the combats...
U4. 4
hippodilemons* --> if this combat happens, the hippodilemons' strategy is to reach bite/grapple and drag PCs off of the boat, underwater, in different directions (that's what the four adjoining caverns are there for)
U6G. puzzle /
demons of iubeni*, up to 5 --> I suspect for your group the puzzle (and that 28 necrotic damage) will be more challenging than having to face a demon one-on-one, but if they're depleted or a PC gets unlucky, this could wear them down for upcoming combats in the Osirium
U9.
serpent of the Duat* --> I'd probably have the serpent have previously cast its
symbol somewhere on the "island", and open with
wall of fire around either the PCs' boat or the "island", so that they remain clustered for its Fire Breath and when it does start grappling it can pull them through the
wall of fire
U13. 4
ophidiles* --> Frog God set this up as an ambush encounter with astronomically high Perception required to prevent being surprised, the ophidiles (between constricting/grappling & paralyzing spittle) can definitely mess a party up, but it heavily depends on them gaining surprise, so if your party has a
weapon of warning or other ways to nix surprise, it's probably a lot less dangerous