First off, let's get some context out of the way. Throughout Homestuck, Purple Bloods refer to something called The Dark Carnival. However, it's only during Karako's route in Friendsim that we actually see it first hand.
https://youtu.be/JWaVMOGblMY (relevant info starts at 15:59)
Karako and the Reader get harassed by Seadwellers. In the Bad End, Karako fights them and dies. Then, The Dark Carnival appears and takes him away to the after life. It's described as "entering the dimension Alternia occupies" and the mere sight if it it enough to give the Reader an existential crisis because of it's sheer size.
Worth noting is although this is the Bad Ending, the events if it are still canon. Doc Scratch simply rewinds time every time the Reader messes up. The Reader even remembers all the Bad Endings after getting his memories back in Pesterquest and can still faintly remember them throughout Friendsim proper.
Then see how we meet "Karako" in Pesterquest.
https://youtu.be/jBPCWL0xTi4 (14:04 onward for relevant info)
Feferi calls her advisor to get advice from after talking with the Reader about starting a rebellion. This advisor turns out to be "Karako", who the Reader recognizes. He also recognizes the place they meet him in as being The Dark Carnival. However, he also recognizes "Karako" as being distinctly off.
He doesn't recognize the Reader, his mind is strictly on task rather than the more childlike and curious Karako seen in Friendsim, and he doesn't even change expression anymore. Even though, if you choose not to transcend, Feferi dismisses everything he said as "a bunch of bullshit", that doesn't change the fact that she and the Reader still undergo a transformation in the Transcendence ending.