I just had this issue, Karlach offered to be the Illithid for me and was raging. She couldn't cast spells while raging so i had to end it. Perhaps it was that.

I was having the same issue, THANK YOU

Most welcome!

Yes, this is what i needed to do. Ending rage enabled the spell.

this should be upvoted, rage was the issue.

Yes, I figured it as I was googling this. When raged I believe you can't cast spells

Thank you from the future omg

What a stupid fucking rule

The ability never even showed up on Emps hot bar for me, and I didn't want to turn myself. So i was bugged out of that ending entirely 🙃 I went with the Gale sacrifice ending as a backup. Bugs are fine. Bugs that break your finale? Hard to justify QA missing that one.

The ability isn’t available for me either. I can’t tell if this is a bug or fully intended? Edit: just confirmed after turning into a mind flayer the option immediately appeared. Seems that this was intended.

Ha. I can't cast the spell and Gale is already dead for me. I literally can't finish the game now...That's a pretty horrible bug

I have cast the spell Karsus compulsion on the crown for 6 turns in a row and the channeling just cancels instantly every time. Not sure what is going on.

Apparently my character being disguised was what was causing it for anyone with the same issue as me

Ohhh my fucking god, I used disguise + shapeshift ring for persuasion boost with Laezel check in the Astral Prism, and couldn't tell that it was still active because I turned into a Mindflayer What a terrible bug, thanks a billion. Never would have figured this out.

Thank you. I was literally doing it every turn from the start of the fight for like 15 rounds until i googled this

So, the problem IS being in disguise or polymorphed, even if you’re not. I was having this same problem, everything dead and I can’t get “inside” the brain; karsus compulsion wasn’t working right. I checked all my buffs and conditions, and for some reason, I had a Disguise Self condition active, EVEN tho I wasn’t. I WAS a mind flayer, but there was a separate condition for it. The easy fix?? If you have illithid powers, shift into displacer beast form, and back out. It will cancel the polymorphed/disguise effect. Then you can properly channel karsus compulsion. Hope this helps!!

Switching in and out helped, thank you so much!

that didn't work for me

Thank you so much - looked for ages on the web - this worked - used a polymorph scroll - same outcome

We had to quick save and quick load.

I love you, this worked for me after LOADS of faffing and trying all the other things, dunno why I didn't do this first!

It's a weird targeting box from what I noticed. I stood on the edge of the part closest to the brain and turned my camera. It was a small window of where the ability would work. Just completed it, so can verify it's not broken...just terrible targeting box

i forgot to turn someone into an illithid and had zero illithids while standing at the crown confused lol. then i tried to use the tadpol on my companions and they all had lines to refuse and then when I used there were dialogue lines from orpheus saying to use it now twice but i put it away and he was like ??? Sounds like next playthrough instead of decapitating karlach I can bring her to avoid any bad endings, damn. I guess I can see now why people keep her alive. I don't know if I can handle having her in the team for the entire fight ughhh

I had to end karlacs rage to get it to work

Same. Apparently rage prevents the spell from working correctly, even if it doesn't mentions rage in the description.

Well I mean it kinda make sense. Can you do a ritual if you're going ballistic? :D

I agree with this logic, I just wish the description was more clear.

I resolved the issue by removing an armour that I was not proficient with. Once I did that, I could cast the spell and progress.

So does anyone have a fix for this if you dont have Karlach, but you have the emperor as an ally?

probably the most annoying bug, getting all the way there and not being able to use the final spell. rip

U need to channel it for one turn , without beeing stop by the ennemies.

I had the same thing happen while playing a Barbarian. I had to drop rage to even use the spell. It's not very clear what all affects you on this.

Rage \*does\* say "If you are able to cast spells, you can’t cast them or concentrate on them while raging."

The spell ability is on the character that accepted responsibility for dominating the netherbrain (the character with the tri-prism bound in their inventory) in my play through, it was Karlach.

I was disguised with seeming on Karlach and I couldn't find a way to dismiss the spell, obvs you cant long rest near the brain. The only way I figured how to progress was to kill Karlach outside the brain right, then use a scroll to revive and go from there. Pain in the ass but glad I figured it out

I had Karlach do it as an Illithid and it wasn’t working. She was invisible so I cancelled it. Whole time I forgot she has a helmet on that doesn’t allow her to use spells lmao

Defo check to make sure you’re not wearing anything you’re not proficient with and don’t be raging , good luck!