I think I don't know how to run challenging encounters
Yesterday evening, my players fought the BBEG of the first act of our campaign: the cult leader of a group of depressed dudes worshipping a big ancient evil snake… Nothing very fancy, but still, my players enjoyed it.
They said the atmosphere was great, they loved my narration, and they praised all the interactions I gave them with NPCs for the final session. But the fight itself felt very plain.
I know I’m more of a roleplay-focused DM, but I felt really disappointed that the conclusion of a 15-session arc was that easy.
To sum it up, the fight was very long, boring, and not that threatening. The “biggest” move we (the players and I) remember is when I cast Banishment on the cleric (the only healer), and that was it.
The fight lasted almost 3 hours. Three hours of attacks that felt like white bread, three hours without any real tension. It got to the point where even I joined my players in joking about their opponent.
Just to give more context, my party consists of four level 6 characters: a Life Domain cleric, a Great Old One warlock, an Assassin rogue, and an Evoker wizard.
So, I’d like to know if anyone has ever been in the same position. Is there some kind of math I should be using to build the perfect balance between hit points and damage output? Or is it just that I gave out too many magic items/potions ?