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.
Fireball (8d6) 27 avg damage per target per round
Greatsword x2 (4d6+20) 34 avg damage per target per round
Longbow x2 (2d8+20) 29 avg damage per round
Your players do around 30 damage per round on average, excluding crowd control and the spread of AOEs. If there are 7 players, you can expect 210 damage per round, +- a bit. Your 300hp monster will last 1.5-2 rounds.
*Strike from ambush so it gets a surprise round
*maximize its HP to about 450
*Give it legendary resistances
*Rework the terrain so the party can't keep it from reaching them, and so it can reach the squishies. Multiattack the Wizard who has Reverse Gravity, not the Fighter. Also give some spots where nobody wants to go, like ponds of murky water or (concealed?) pit traps. If someone's near a trap and between the trap and the enemy, sub in an Athletics check for one attack to shove them into the trap.
*Give it minions. Six cultists spread out around the room, each hiding behind improvised barriers or statues (3/4 cover, which includes a bonus on their Dex saves against fireball), casting Hold Person, Faerie Fire, Bane, Guiding Bolt, etc. at the party. For your convenience, 3 of them cast Bane and 3 of them cast Hold Person, then the next round 3 do FF and 3 do GB, etc.
This is mostly the same advice given back on page 1, just specific to this encounter.
You are the change you are looking for.