On Tuesday one of my DnD groups went up against a vampire. Vampires seem pretty nasty in 5th edition, and this particular one was a CR13 while the group was only level 8. That would be fairly dangerous on its own but vampires have regeneration as one of their abilities and that makes any level disparity much more important. If the group can do 40 damage / round and the vampire regenerates 20 / round, we do a net 20 / round. If we are a bit underlevel though and only do 30 / round, the regeneration reduces that to net 10 / round, and it takes the vampire twice as long to die.
This vampire should have annihilated us. It wasn't just a CR13 vampire because it started in a zone of magical darkness that we couldn't see through but it could ignore because it had truesight. It also had a tremendous advantage due to terrain. It resided in a 30 by 30 room that had a 5 foot wide hallway leading to it. Our group has 3 melee characters in it so the vampire could just park itself in the hallway and beat us senseless. If we stay away from it then its regeneration quickly returns it to full health, and if we close in then only one or two of us can attack and that isn't enough to overcome its regeneration while it mauls us.
The vampire didn't need to use the hallway to win the positioning war. It has 3 legendary actions per round, each of which can be a move that does not provoke attacks of opportunity. All it has to do is fight us for a bit, then run away and hide on a ceiling out of range. We can't escape because we would have to make a terribly dangerous trip across boiling mud, but since the vampire can walk on walls and fly it could happily regenerate to full and then come back in a few rounds to eat us. This is even more effective because our barbarians lose their rage if the vampire retreats for a round and once they aren't raging we get absolutely smashed.
Even then it should never need to mess around with positioning at all and can simply beat us with raw power. The vampire has a Charm attack that has a 75% or more chance to succeed against 3 of our 4 party members. If the Charm hits we are out of the fight completely. The vampire can simply use its legendary movement actions to zoom away from us and fire off Charms. We rate to lose 2-3 party members to the Charm, and it doesn't even matter which ones get hit since no 2 characters stand a chance.
There are some ways characters can respond to these problems. You can use Dispel Magic on the darkness that the vampire starts in. If you don't do this you are *completely* boned and you will lose. The fight is already extremely dangerous numerically, and stacking on advantage on all of the enemy's attacks and taking disadvantage on all of ours makes it impossible. Either have Dispel Magic, or lose. We had Dispel Magic though, so we had a chance.
You can use Protection from Good/Evil to prevent the Charm attack. We didn't have access to this, but the GM didn't have the vampire use the Charm until it was already badly hurt. If the vampire had just used the Charm right away, we lose.
So this opponent had four different things it could do to flat out beat us. It had darkness, constrained positioning, incredible speed, and its charm ability. We had the magic bullet for the darkness, but it didn't bother using any of its other three auto win abilities to crush us. Whoever wrote this mess either didn't bother to think about what the monster could do or assumed the GM would not maximize its abilities. I get people not thinking about what incredible movement speed and regeneration does to an encounter, but not looking at the Charm and darkness and realizing that they absolutely end the encounter unless you have a specific spell is sloppy and embarassing.
I particularly hate the Charm ability as written. Most crowd control effects in fifth edition let you make saves each round to get rid of them. People remember how awful it was to get hit with Hold Person in old editions of DnD and then just stand there for an entire encounter doing nothing, (Roleplaying being paralyzed loses its fun after about six rounds of it...) but apparently they are okay with handing out abilities to monsters that do that same thing. If a fight will take five or ten minutes I am fine with knocking people out of it early, but a fight that is designed to last a huge amount of time should not work this way.
The last thing that frustrates me about the vampire is the same thing that frustrated me about half of the monsters in this dungeon so far - they take half damage from non magical weapons. I get the story driven thing about how you want to make a monster sound exciting, and saying that normal weapons just aren't effective can do that. But when you stack it on half the monsters it makes the game feel stupid. I should have chosen a magical weapon. It would make me do double damage in many situations because of this trait, and 20% more damage generally. Had I realized that the high level game was designed for everyone to have a magical weapon I would have done so.
I am having fun with the group I am playing with, but good grief I am glad I am not trying to actually roleplay because this dungeon is an absurd mess. It really makes me feel like I should write up some dungeon crawls and do it properly.