Actually, Wish should be the only possibility... I went back to read Sul stat block. She doesn't need Legendary Resistances to prevent being successfully bound. Planar Binding takes one hour to cast, that's 600 rounds. Casting spells with a time of more than one action requires concentrating during the whole casting. Interestingly, Sul Khatesh has 3 legendary actions on top of his legendary resistances.
- Attack. Sul Khatesh makes two attacks with her Arcane Blast or one attack with her magic staff.
- Consume Magic (Costs 2 Actions). Sul Khatesh targets a creature within 120 feet of her who is concentrating on a spell. The target must succeed on a DC 26 Constitution saving throw or its concentration is broken on the spell, and Sul Khatesh gains 5 temporary hit points per level of that spell.
- Maddening Secrets (Costs 3 Actions). Sul Khatesh whispers an arcane secret into the mind of a creature she can see within 60 feet of her. The target must succeed on a DC 26 Wisdom saving throw or expend one of its spell slots of 3rd level or lower and deal 26 (4d12) force damage to each creature within 30 feet of it. A creature that fails the saving throw but can't expend a spell slot is instead stunned until the end of its next turn.
Notice that Maddening secrets targets "a creature she can see", while "consume magic" doesn't have the limitation of being able to see the creature. She is aware of any spell being concentrated on within 120 ft so she can consume it.
So, the attempt goes like this:
Cheating One: I sneakily approach Sul Khatesh in her planar prison that I found... in a box of Cheerios.
DM: OK... I have no agency, what should I say?
Cheating One: then, I cast Planar Binding on her.
DM: OK, on round 1, just after your action, Sul Khatesh uses her ability to Consume Magic... she hasn't eaten a spell for millenia, she's hungry... roll a CON save, DC is 28.
Cheating One: I rolled a 32.
DM: No, you can't do that on a D20.
Cheating One: That's DM fiat to tell me how many sides my d20 has!
DM:...
Other players: Look, you have a +7 modifier to your roll, the max you can get is 27, even on a result of 20. That's failed. You can't resist the maddening power of Sul Khathesh feeding on magic around her.
DM: Exactly. Sul Khatesh gains 25 temporary hit points ; your spell is lost. I'd rule that SK feels the rush of adrenaline as she absorbs the magic but since I have no agency, I guess I can't decide that... maybe she's still unaware she's just taken her breakfast.
Cheating One: OK, I cast Wish and replicate the 8th level Planar Binding effect.
Other players: Sul Khatesh has 95% chance of succeeding on the throw, but even if she fails, she'll use a Legendary Resistance.
Cheating One: OK, I'll cast other spells at her to make her expand her LR before!
Other players, awed by such ingeniosity: Stupendous! Wonderful! Creative!
Cheating One: let's review my spell list....
1 - Armor of Agathys, Absorb Elements, Mage Armor
2 - Aid Spell, Pass without trace
3 - Counterspell, Dispel, non detection, Eruption Earth
4 - Dimention Door, Banishment, Polymorph
5 - Seeming Spell, Wall of Force, Skill Empowerment
6 - Freezing Sphere, Disintegrate, True Seeimg
7 - Etherealness, Reverse Gravity, Teleport, Plane Shift
8 - Demiplane
9 - Wish
So, Wish is out because I need to cast it later. My 1st level spells are... defensive. My 2nd level spells are... defensive. Counterspell doesn't require a saving throw and SK isn't casting a spell... I could Dispel Magic at her but she has no magic except those strange draconic manacles (other players vehemently veto trying anyway)... Erupting Earth does... 3d12 damage and SK has 475 HP. She won't bother to use a LR and casting it would reveal I am casting spell at her... Dimension Door is Self, Banishment would release her from her planar prison so she'll voluntarily fail her saving throw, Polymorph... Yes! She will have to resist polyorph... ah, drat, the spell doesn't work on shapechanger and she can change shape at will... None of my 5th level spells are offensive spells...
Let's use the big guns then... My six level spells, she will
have to use a LR. Freezing Sphere does... 10d6 on a failed save! Drat, she succeeds naturally on 75% of the CON rolls so it's not sure I'll be able and it reveals my position because of the frigid globe of cold energy. Disintegrate... is a thin grin ray and she can stomach 10d6+40 anyway... None of my higher level spells can hurt her!
Other players, tired of the monologue: Listen, you
knew deep down, that wizard were superior with their ability to memorize spells adapted to the task at hand if given a day of prep'...
Cheating One: NOOOOOOO I'll cast repeatedly Disintegrate at her, DEX is her weakest saving throw, she needs 12 or higher to succeed! I can cast it twice at level 6, once at level 7 and once at level 8... that's a total of 4... She'll need to use her legendary resistance or face an average of... 55% chance of getting 75 HP of damage. So basically she'll... never spend enough LR to counter threat before I run out of magical fuel. Plus, it will take 4 rounds, during which I'll be totally visible at the sore end of my Disintegrate ray. Can I survive FOUR ROUNDS of SK in order to get my 9% chance of making her expand all her LR? That's if she doesn't use Arcane Cataclysm, or even her regular 4 ranged attack a round.. her DPR is 60 against the sorcerer. He'll be dead before he could finish casting.
So, basically, there is only Wish to try (5%) to Planar Bind Sul Khatesh. And Wish can't be cast after forcing her to expand LR because she can reduce the Cheating One to smithereens before she's forced to do so.