D&D 5E Would making powerful enemies immune to cantrips make the game more or less fun?

[grimslade]

I see this as being roughly equivalent to requiring martial characters to use silvered or cold iron weapons in certain situations. Which is perfectly fine, as long as those characters have a way to get the right weapons eventually or have something else important they can do in a fight.
If you're going to do this, I'd suggest you also give the PCs a way around it. Maybe PCs that use particular spellcasting implements have their spells do full damage?

This is an interesting tack to take. What if you required certain foci to cast spells at the creature? You need the Book of Ashanti to cast spells/cantrips at Demon Lord Zrag'gt. Pennythimble, Marquis of the Unseelie Court requires a cold iron wand to break his spell immunity. This makes the challenge able to be overcome and gives some life to arcana checks and research. Alternately, the True Name of extraplanar creatures could lower their defenses.

 

[toucanbuzz]

I did something like that for the named Demon Lords in Out of the Abyss: they're immune to 2nd level and lower spells unless they want to be affected, immune to nonmagical attacks, and anything less than +3 weapons or another demon lord-type attack, they're resistant.

It may sound harsh, but they're supposed to be badass eons old demigods that have survived the Blood War and intervention by gods. When a group of PCs run along after roughly 6 months of on-the-job training and confronts one, it seems silly they'd take them out in a typical 3 round D&D combat.

In short, I'm a big fan of using certain AD&D-style monsters more like puzzles to be solved than a big bag of hit points to be whittled down. It gives individual classes a chance to shine and can lead to innovation, such as the magic-immune iron golem being tricked into stepping onto an illusory bridge, or clever use of a Dig spell (created a big pit) against a 90% magic resistant creature. Sometimes you had to outthink rather than outpummel, and you had to take special care to collect a variety of weaponry to overcome resistances.

Side note: I ran the finale of Out of the Abyss by giving each PC a demon lord to control, then let them fight it out. The PCs then took on the survivor, a weakened demon lord but a very serious threat nonetheless. There was no warlock character at the time, but in future campaigns, I'm using the A5E version where eldritch blast is a class feature, not a cantrip, solving any future issue if we run into Demon Lords.

 

[Composer99]

Side note: I ran the finale of Out of the Abyss by giving each PC a demon lord to control, then let them fight it out. The PCs then took on the survivor, a weakened demon lord but a very serious threat nonetheless.

That sounds like a blast; if I ever run that adventure, I'll be doing that!

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On topic, I think that a sparing use of cantrip immunity would be more fun, probably as long as there are diegetic ways of bypassing it à la silvered weapons for lycanthropes.

Overusing it? Definitely less fun.

 

[Irlo]

I have a few times used creatures that could reflect cantrips back onto the caster (usually on a reaction, sometimes triggered by being targeted or else on a missed attack roll/failed saving throw). The ability didn't come into play much, and I'll always regret that the Minor Boss never got a chance to send back a Vicious Mockery while shouting, "I know you are, but what am I?"

To answer the question at hand, I think that immunity to cantrips would add to the fun if used sparingly, but, if implemented as a standard ability for high CR creatures, it would not be fun.