Parmandur
Book-Friend
It always steams my buns a bit when they do the whole "boundaries between worlds are breaking down!" comic book schlock.
Like, okay, Tomb of Horrors is a classic, and yeah, you want to update it, so keep it in Greyhawk, you donks!
If you have to stay in FR for some arbitrary branding reason, you should double down on what makes FR exceptional and unique, like no other fantasy property. Cribbing stuff from Greyhawk doesn't do that!
Tossing chunks of GH into FR is lame. It feels inauthentic. Stop trying to turn yourself into something for everyone FR, just be yourself and let GH be itself, it's fine to not have everyone love you.
Maybe Ackerack is the villain in an upcoming AP: "Secret War on Infinite Planes in Realms Shaking Crisis"