I'm starting to think we don't need spoilers for a thread dedicated to enhancing the adventure- and whose title is marked with spoilers
Miska the Wolf Spider is CR26, but the adventure doesn't have the demon lord as an enemy normally- Miska can only use legendary actions while they fight a CR13 and a CR19 bodyguards. It does say that if the DM thinks that the players are very skilled, they can fight this "deadly" foe- though Miska will still be immobile. At this point the characters are level 19.
Miska is a damage sponge with 400hp, but his three attacks deal embarrassingly low damage for CR26. His most powerful features are his spells, which involve DC21 dominate and suggestions- but at level 19 I'm thinking most parties would make mincemeat of this?
This whole encounter is instantly defeated if a character makes a DC25/DC10 Athletics check to pull a piece of rod from the floor (DC depending on an earlier event). Miska is sealed away and his spider-fiends flee.
Hm.
I'd think that Miska has such a reputation that they should be something of a threat- upping the damage of their attacks and adding some debilitating effects could help. It seems that Miska's spells are all about domination, so maybe there's a poison on the trident that also gives disadvantage on saves vs charm or wisdom saving throws?