As well as sweet innocent creature becoming an unholy eldritch abomination of NOPE!
I gave a rather large list of XD's in that fanfic. From XD-004 or Shadow Solgaleo who is an important plot relevant shadow pokemon. (And showing some growth in Lillie when she is the one to snag it.. and SHE IS NOT HAPPY ABOUT IT.)
XD-005 or Shadow Nihilego who did little before being snagged, but brought an Ampharos Wes has to the verge of insanity/ death from a modified version of the neurotoxins. (Design to kill you faster as throw away Cipher scientist demonstrated when he merged with it.) As well as being the Nihilego who was milked for fudged up experiments done on Mew
Then XD-002 or Shadow Mew who, even though I kept the really horrible stuff as back story/ build up. I made him special even among them (He has transform.) I have him take on the from of everything Elio (P.C) Lillie, along the others dragged into that mess, had fought through out their lives in an attempt to kill them.
Pretty much this boss.
Delirium from the Binding of Isaac Afterbirth +.
From Totem Wishiwashi who, what ANYONE who played S/M will agree on, was a pain in the butt, to more personal forms like Type: Shadow. (A shadow pokemon version of Silvally.) or an even warped version of Mother Beast Lusamine. That Mew knows enough about our heroes just by reading their memories and will strike at them on a personal level. A big bonus since it unleashed hell all over Alola and resulted in the background characters fighting back against a mind that is gone and a world that is undergoing a glitch. (When I said it seeks the destruction of all, I am not sugar coating it.)
Then Shadow Mew exploits a loophole at the moment of its defeat to break free from Ardos's control, brought Elio to heart breaking insanity. (Forcing just a small image of what the Mew had gone through along the feelings of what was done.) All in an attempt to end it all with a shadow explosion. (That would have wiped Alola off the face of the earth and that isn't even the scariest implication. Mew attempted suicide.)
That is when Lillie finished charging up the Master Ball and got an aim to use at the Mew to put an end to the insanity.
Elio ended up in a coma for a week and when he woke up... he was in tears and left with a SEVERE case of PTSD. (A lot for a twelve year old boy to go through.)
The rest of the Fanfic was Mew having all of that crap removed from his and an epilogue arc.
Shadow Mew's plot beats seem pretty familiar to another villain we know of. A character that had one mentally scarring effect happen to them followed by another that made it worse, when that character is met in story, he/ she was at the peak of insanity and was stopped at the last moment from carrying out the point of no return, someone ends up in a coma. (Instead of the villain this time.)
Then that villain gets taken to another region for treatment in hopes to salvage the soul.
For the rest of this series... Shadow Mew gets mentioned a lot and even reused a few times. (Second attack on Elio, a nightmare and as a way to compensate for NOT using an alternate Lusamine in the RR arc by using a Mew from another reality if he was able to go through with blowing himself up "They are rather similar in terms of plot beats anyways, it is a fair trade.".)
But that will be for another day and I might top out Shadow Mew... might.