@Niara The CR system is messy when it comes to certain things. [...]
For a campaign you don't have to adjust anything and could simply pick an enemy from a higher challenge rating or you can severely limit what rewards a party can attain or you can make some changes. Regardless an enemy from the campaign can, but does not have to, follow the same rules to the letter as players.
I'm well aware; CR is a guideline, and has been moderately carefully balanced using a set of rules that add up and calculate a creature's offence CR and defence CR, and along with other modifiers work together to create the final projected challenge rating for the creature - being a projection of a moderate but not overwhelming threat to a party of between three and four player characters of a level equal to the projected CR. I am very well versed in all of this, and use it along with the necessary adjustments to make custom creatures quite regularly as and when I need to for a game.
Because CR represents a moderate threat to a party, a well rested party with good synergy or good planning can usually punch substantially above their CR, at the cost of more of their daily resources, but at relatively low overall danger. This is normal, and a good DM accounts for it without punishing the possibility that a group won't be as tactically organised. There are, of course, a number of specialised cases of individual creatures or certain scenarios where setting a higher CR monster can be disastrous for the group, even if they'd normally be able to punch at that weight on another creature. Spell access vs. spell immunity by level is one such most notable case (it's even highlighted in the DMG as a classic case where this kind of outlier crops up), and a good DM watches out for these situations when designing encounters or areas. Please trust me when I say I know what I'm talking about.
In a video game, our DM (Larian) as ABSOLUTE control over exactly what we are able to see, where we are able to go, and what we can fight, when... they have no player anomalies to account for because they control, absolutely, what we can and cannot do or be, and what we can or cannot try. This means that they have zero excuse for lazy hack jobs that unbalance encounters, or for cheating the rules unfairly because they haven't wrapped their heads around proper encounter design; unbalancing a creature by giving it far more spell access than it should, or even *Could Possibly Ever* have, without adjusting its CR, is just ridiculous, lazy and stupid, and it pushes players towards cheap, cheaty strategies and immersion-breaking cheese. Of course, we've already seen that that's what Larian wants, because that's what their developers define 'fun' as being.
The key point here is that the Archmage is a CR 12 creature but an 18th level caster. So in BG3 it'd be given the designation of "level 12" but would be able to cast 9th level spells, something no level 12 player can do.
It should either display as Level 18, or as CR 12. Anything else is Larian's failure and not something that we should account for or accept... I know this is how they do it right now, and it's not an excuse I accept. Calling such a creature Level 12 is just Larian showing that they have not understood the system they're working with yet.
It doesn't matter what internal level or external CR a caster character is, it is never going to have eight 1st level slots, six 2nd level slots and four 3rd level slots; that's not how magic works in 5e. It is a hacked, cheated character that is breaking the game's rules, and it's a poor excuse, and a lazy one, to actually designing fair and challenging encounters.
