Maxperson
Morkus from Orkus
They show up, because since 1e certain specific named spells have existed and travelled to other worlds. Had it been Mordenkainen's Mansion or something, I wouldn't have an issue with it. However, if you're creating an entirely new named spell for the Forgotten Realms set...Mordenkainen spells shoe up in the Forgotten Realms, part of the whole multiverse thing: in fact, I wouldn't be surprised if Mordenkainen is in the Set as a Planeswalker.