after all is said & done, you'll end up a skeleton

for the king in yellow himself, he is 100% gone. because of this, derek's memories are 100% gone. derek will never get them back, but the king will never come back. they were both destroyed (or as destroyed as a cosmic entity can be, but if i get into that i'll be wholly off topic)

for his memory, he sort of has muscle memory, but all his old skills and stuff are mostly gone. he'll have an easier time relearning things he used to be good at (like, he can probs pick up programming and stuff at a glance / without needing to really study it) but he WILL have to learn it again

as far as physical effects go, the only lingering change in derek's physical appearance is very, very slight yellow in his eyes. this is a "can only even be seen under just the right conditions" sort of thing, and it'll take avery a long time to notice. the only other real lasting thing is occasional dreams of the places he saw while playing, but he doesn't really coherently remember them when he wakes up

for mental changes to derek, we were thinking of all effects inspired by the OG KIY novel and the king's usual interest in / ties to theater and artists. derek is now a much better liar(/actor) than he used to be (though avery can still see through him). he also has the ability to paint now (or. pick it up quickly like his other skills. not that either of the are aware this is a new skill). i like the idea he one day paints some weird landscape he thinks he might've seen in a dream once and avery nearly has a heart attack bc derek has painted something from the world

(yes this is a little bit hp lovecraft the call of cthulhu inspired. it's fine)

the really big thing (and the only one that might come up, somewhat, in the fic i've committed to) is that derek cannot interact with anything related to the king in yellow or that world. watching the footage makes his nose and eyes bleed. touching a copy of the kiy novel makes him faint. avery can tell him about it to an extent, but direct quotes from everything would give him a headache. i think hearing or reading the actual ritual or any of the king's words would be excruciatingly painful. his dreams are the exception, otherwise, this is a gaping wound that will never heal. messing with it has consequences. they will find this out the hard way.