Okay...actions.....happy to tone down his staff attacks if that's what's needed to balance.
Well it's already toned down slightly because we reduced the STR bonus by a point.
Let's see, if he makes three Multiwhacks with its Staff for 2d6+7 plus 2d8 cold (23 times 3 is 69) plus a Bonus Action Mockery for 4d6 (14) and a Legendary Action Cast a Spell, say
cone of cold for 8d6 (28) that's… 111 DPR.
Which is Challenge 21 according to the Calculator.
Indeed it's the very lowest end of CR 21, we can have a range from DPR 111 to 122 and it comes out to that figure.
So we
could increase the damage a tad and keep within budget.
However, I'm thinking we should drop the Cast a Spell Legendary Action since none of the Archdevils or Demon Princes in Mordenkainen have it as far as I can see.
Instead I'd rather, say, increase the Vicious Mockery to 6d6 and be able to use THAT as a Cost 2 Legendary Action. That's the same average damage (6d6 + 6d6 instead of 4d6 + 8d6).
Oh, I'm thinking we should adjust the Innate Spellcasting so it has some at-wills like the 5E Titivilus and Hutijin, who have:
Hutijin
At will: alter self (can become Medium when changing his appearance), animate dead, detect magic, hold monster, invisibility (self only), lightning bolt, suggestion, wall of fire
3/day: dispel magic
1/day each: heal, symbol (hopelessness only)
Titivilus
At will: alter self, animate dead, bestow curse, confusion, major image, modify memory, nondetection, sending, suggestion
3/day: greater invisibility (self only), mislead
1/day: feeblemind, symbol (discord or sleep only)
The original 1E versions had:
One at a time, at will, 1 per round, Hutijin can use the following spell-like powers as a 20th-level magic-user: animate dead, detect invisible, detect magic, heal twice per day, hold monster, invisibility, know alignment, polymorph self, produce flame, pyrotechnics, ray of enfeeblement, read languages, read magic, shocking grasp, suggestion, teleportation, wall of fire, fulfill another’s limited wish. Hutijin causes fear by voice tone within a 30 foot radius with a saving throw vs. rods et al, applicable. He can use a symbol of persuasion once per day.
and
It is said that Titivilus possesses a silver sword of wounding; however, he typically uses his other powers. At will, 1 per round, as applicable, Titivilus can use the following spell-like powers as a 20th-level magic-user: animate dead, bestow curse, charm person or mammal, chaos, emotion, feeblemind once per day, forget, fumble, hypnotism, illusion, illusionary script, improved invisibility, know alignment, message, misdirection, nondetection, polymorph self, protection from good, suggestion, teleport, tongues, ventriloquism, whispering wind. Once per day Titivilus can use a symbol of discord or sleep (19 levels or hit dice or 99 hit points affected. A saving throw indicates only nodding and dozing with a 1 in 20 chance per round of awakening fully). He causes fear by touch only.
So the 5E version is pretty similar in general outline.
Note that the fear causing effects are Traits or Actions not spell-like abilities. 5E Titivilus has a Frightful Word action he can use separately or as part of a Multiattack with a single sword attack, while 5E Hutijin has Infernal Despair.
Adonides causes fear by pointing at someone once per round, so we could make that a Bonus Action or Multiattack option.