I need some help to make a big bad boss

[Trandir]

This is for pathfinder 1e. I am making a big monster to top off a dungeon. CR 21 and with a nightmare theme.

Some stats that can be adjusted if need be:

- AC 39, flat-footed 35, touch 9
- To hit bonus: +35 (routine of 4 attacks, 2 claws, a bite, a tail attack)
- Special attack: two claws have energy drain (2 levels DC 33)
- Saves: Fortitude +19, Reflex +23, Will +24
- Perception: +38
- HD: 26, HP: 420
- Resist: fire 25, Cold 25, electricity 25, acid 25, sonic 30
- DR: 18 good
- SR: 33
- Immunity to mind-affecting effects
- Senses: 180 ft true vision, 60 ft blindsight
- Special ability: continuous air walk

I also wanted to give it some nightmare and fear themed SLA but I am having some difficulties with the SLA. Any advice would be greatly appreciated

[Kurald Galain]

A suitable pick would be Phantasmal Killer (as a swift action), or Weird (its area effect equivalent, as a standard).

Also, it strikes me as fitting to give this guy permanent Displacement.

[Trandir]

A suitable pick would be Phantasmal Killer (as a swift action), or Weird (its area effect equivalent, as a standard).

Also, it strikes me as fitting to give this guy permanent Displacement.

Also about 34 DC?

[Seto]

Well, if we're building the monster following the usual rules, the DC for a Phantasmal Killer SLA would be 10+4+Charisma mod. It's probably gonna come down to quite a bit less than 34.
Now if it were a special ability, that'd be 10+1/2HD+CHA mod. You have 26HD, so you can get to DC34 with a 32 CHA score. What about making it a special ability that triggers under certain conditions (as in, "if the monster establishes a pin on a character, that character is subject to a Phantasmal Killer effect, as the spell", etc.)?

Also, with a nightmare-themed horror, some kind of Horrific Appearance (Su) seems in order, as with Qlippoth. If you wanted to be mean, you could combine it with the Phantasmal Killer to make it a one-time Mass Phantasmal Killer as an opening to the fight (standard action -> all within 30ft. get a DC34 Will save -> those who fail get a DC34 Fort save against dying of fear). Or you could give it a Horrific Appearance with another, more long-term effect (2d4 Wisdom damage and frightened for 1d6 rounds?)

For this kind of fight, you really should know your party, though. As in, how likely are they to pass a DC34 save? What defenses do they have available against fear and mind-affecting, are some of them immune? That'll make a huge difference in the fight.

[Trandir]

Your speciality seems a bit odd.

What do you mean?