Ilbranteloth
Explorer
I hate to interrupt the discussion of Gods and Powers, but I had a discussion with a friend (a huge FR buff) who suggested something interesting...
We were talking about Curse of Strahd and, how except for minor differences, Strahd is just a MM spellcaster-variant vampire (with a higher Int score a few different spells) and Mordenkainen was a MM archmage (rather than a unique NPC stat block). Accerack in ToH is a MM demi-lich as well (with his variant power in the MM). It appears that, with a few exceptions, WotC ok with not custom crafting stat blocks for NPCs and is willing to use MM stat blocks for famous heroes and monsters. This lead us to a thought about how other "NPCs" in the Realms might now be as powerful as they were made before.
What if, Elminster is an MM archmage rather than a 30+ level fighter/cleric/thief/wizard/archmage that 3e had him as? What is Artemis is a 16th level rogue (assassin), Drizzt a 16th level ranger (hunter)? The Seven Sisters weren't epic level but Challenge 12-14 NPCs?
What if the "epic demigods of FR" aren't so mechanically epic anymore? That a group of mid-levels PCs could take out Elminster or Drizzt? They are high levels for sure, and Elminster is still capable of 9th level magic, but They aren't the Epic Level Gods of 2e or 3e anymore? (Most of their story exploits being owed to plot-armor and convenience) Thus, we can answer why can't Elminster go and solo Tiamat in Rise of Tiamat? Well, can a challenge 12 take out a challenge 30 solo by himself? Grab the stat blocks and try it. Hell, give Elminster help from Kelban and Allustrial (MM archmages) as allies and have them take on Demogorgon (Challenge 25, OOtA) and see how that combat plays.
Once you frame the notion like that, I wonder if it changes the notion of "why isn't Elminster stopping the Underdark invasion of demons" when he's more limited to what a 20th level wizard can do rather than a 30th level rules monstrosity...
So throughout the history of the Forgotten Realms, those famous NPCs have changed level with every edition. What is consistent, and was the original intent, is that many of the NPCs (not just the famous ones), are high enough level that they can't just be attacked by the PCs, and will pose a credible threat if they are a foe. This is consistent with some of the original approaches in D&D, where spells of 7th-level and higher were originally designed for NPCs to allow them to do things the PCs couldn't, and so it could reflect that they were of higher level than the PCs.
So naturally, as PC levels increased, the levels of the NPCs did too. In 5e, I think the majority of the famous NPCs would be 20th level with several boons.
Having said that, all "famous" adventures, that is, the ones not involving our PCs directly, are subject to the effect of bards and their tall tales. So Eliminster from the PCs perspective "may not as powerful as the tales say. Of course, unless ye've actually met him and can prove otherwise. Nothing but the fabrications of those that tell such tall tales and make their coin from it."