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Hot take: Strongest creatures in the setting shouldn't just be clowned by PCs with no resistance.
(ttrpg.network)
Inspired by frustrating conversation I had. For those curious, that was the statblock of Caine, father of the vampires.
To be fair, starting at around level 13 it becomes more challenging to, well, challenge a party without having dragons and shit everywhere. You can almost not build encounters with "normal" enemies anymore.
Before my group got bored with D&D and we decided to give it a break and switch to Mage, it got to a level I was prepping to have the this level 13 party fight Vecna and Zariel at once, just to make it at least a little hard.