I was thinking about how, in older editions, demons / devils and other powerful enemies might have immunity to spells of a certain level. If this was ported over to 5e, do you think it would make the game more or less fun?
I imagine implementing this house rule would mean powerful enemies of certain types would have "cantrip immunity." For example, dragons, demons and devils, celestials, and maybe powerful undead would have it. The idea would be that "common magic" no longer effects these powerful beings.
Obviously this would be a big blow to warlocks, who rely most on cantrips out of any spellcasters. For wizards, clerics, and other spellcasters, it would mean having to burn spell slots instead of using cantrips. This could be tough at the end of an "adventuring day" in which they have used a lot of spell slots!
What do you think? Would this house rule make D&D more fun or less fun for you?
(NOTE: I'm not planning on actually implementing this house rule. I'm just having fun thinking of new ideas.)
I appreciate the spirit of the idea - the devil is in the execution.
However, I think it's just falling into the same trap that 5e does in regards to immunities, vulnerabilities, resistances, Legendary Resistances, and defensive traits in general. They just don't (a) have enough impact (or occur frequently enough in the case of vulnerabilities) & (b) say enough about the monster narratively.
The problem is that there's no fiction here. It's a broad brushstroke to mechanically differentiate a certain power level, I guess. At least that's how you've presented it. "Cantrip" is a power level thing. It's not "cold magic" or "charm magic", for instance. The reason the rakshasa can kinda get away with Limited Magic Immunity is they have a damage vulnerability to "Piercing from Magic Weapons Wielded by Good Creatures"...but even then I would approach the rakshasa differently.
I'd say we want more things that emulate something like the
flesh golem's Aversion to Fire and Lightning Absorption. Ideally, something with both a benefit and a strong drawback.
For example, a powerful devil might have this trait, replacing Magic Resistance:
Ward of Souls. The devil has advantage on all saving throws against spells, and spells have disadvantage to hit the devil. Additionally, the devil is immune to all cantrips. However, if one of the souls warding it is freed by speaking the soul's True Name, casting
dispel evil and good, or scoring a critical hit using radiant damage against the devil, the devil loses its Ward of Souls until the end of its next turn. While the ward is lowered, the devil is susceptible to cantrips, suffers disadvantage on all saves against spells cast by good-aligned creatures, and spells cast by good-aligned creatures have advantage to hit the devil.