I was assuming that Grazz't would be in the Monster Manual and not relagated to another book. If they are bringing in proper nouns into the MM without even having those proper nouns IN the book, I would be dead set against that.
I have no problem with Orcus being the lord of undead so long as Orcus is actually in the MM. Asmodeus is the lord of Hell? Cool. So long as I can turn to the right page and see who this Asmodeus guy is. But, as soon as you start referencing elements that aren't included in the book, now it's just an advertisement to buy that setting guide. "Hey, Aarococra are like this in Athas." Well, what's an Athas and why should I care?
The stats for Grazz'zt might be in the MM. The adventure that features him or the supplement that describes the Abyss where he lives or whatever is probably going to be a different product, though. If the DMG has a blurb on what the Abyss might generally be, then it can have a little blurb on what Athas might generally be (heck, for much the same reason -- no reason some budding DM can't jam bits of Athas into the desert in their home game). The upthrust being that this isn't an advert -- just because Grazz'zt lives in the Abyss and Braxat live on Athas doesn't mean that you then need to go buy the book about the Abyss or the book about Athas. The MM gives you what you need to run the critter, and references the fact that there might be deeper lore if you want to dig into it.
Now, let's not get too carried away though. I don't think it's too much of a stretch to think that a new player might know real world mythologies. If you reference Greek Myths with a centaur (something D&D hasn't actually ever done) it's not a huge stretch to think that someone might know what those myths are. But, I certainly wouldn't presume that anyone who hasn't been a D&D fan for a long time would know what a Domain of Dread, Krynn or Lady of Pain are.
No one knew what a hobbit was before Tolkien described it. No one know what Quiddich was before Rowling made it a thing. That's new lore: making up new stuff. The MM shouldn't presume you know who the Lady of Pain is, but it can feel pretty confident that anyone can understand the concept of a war of evil against evil. The dude thumbing through the book might not know what a Domain of Dread is, but he might know a Bella Lugosi expy when he sees one.
And, again, just as the DMG can contain a brief description of the Abyss or of the Nine Hells or of the Astral Plane, it can contain a brief description of Krynn, Athas, Sigil, Oerth...like [MENTION=98938]DeF[/MENTION]CON-1 said, a few words and a punctuation mark don't take up that much space.
Reference: "
Athas is a world turn desert by arcane magic."
Hypothetical MM: "Braxats are massive, bestial giants who live on Athas and love to hunt those smaller and weaker than themselves"
Reference: "
Metamorphosis is a normal world transformed by the acts of powerful gods and outsiders."
Hypothetical MM: "In the Metamorphosis setting, Jackalweres were created by Grazz'zt from normal jackals and serve him along with lamia."
Reference: "
Planescape is a world where characters travel through heavens and hells to live their beliefs."
Hypothetical MM: "In the Planescape setting, Mariliths, like all demons, are involved in a great war against devils that pits law against chaos."
Engaged newbie player coming to a table with a homebrew setting having read all that and meeting a jackalwere: "What are these jackalweres like?"
And everybody wins.
Yeah, KM pegged me right on this one. I loathed 2e for this. And, yup, I had to take a big old black marker and redact swaths of the MM every time I ran a campaign for a new player. It was sooooo annoying. And, because 2e tried to shoehorn everything into the same universe, I constantly had players asking to play (Setting specific class/race) in my campaign, because, "Well, the rules say that everything is in one universe and accessible from everywhere else. What's wrong with your game?"
I'd much, much rather that the core game focused on a fairly generic overview of the races - tying them to elements within the same book is fine - and keep all the setting specific stuff contained to that setting. Stop trying to shoehorn everything together. It never works for one thing - the cosmologies are just too different. And the balancing elements don't export either.
I would say that this sounds like pretty much the problem that [MENTION=20323]Quickleaf[/MENTION] was talking about with the method presented in this article: "Oh, I've got to redact and say no to jackalweres because people are going to come in assuming their X, and they aren't, necessarily, that."
The article wasn't very good at reassuring me that this won't be a problem in 5e. Mearls said it won't be, but with that example, it would be. So I don't know that they're seeing this from that perspective.