Zardnaar
Legend
If your group is spellcaster-heavy, a beholder will do the trick. Turn on the antimagic field, stay at least 15 feet up, and eye-ray the archer first.
Doesn't the anti magic turn off the eye ray?
If your group is spellcaster-heavy, a beholder will do the trick. Turn on the antimagic field, stay at least 15 feet up, and eye-ray the archer first.
The main ways that creatures punch way above their CR in 5e:
- Ignoring the HP mechanic. If you can do stat damage, inflict exhaustion, inflict a fight-ending condition like petrification, or otherwise just kill your target, you are on the right track.
- Manipulating the action economy. Mostly just control, action denial is a great way to frustrate players. Stuns, massive penalties, etc.
- Outranging the enemy. Being able to avoid reprisal attacks is a huge deal.
Creating a "killer" monster without going over the top is the hard bit.
Firstly, it needs to do one of those things listed above. Being stunlocked is just not fun, as is being constantly outranged. That leaves ignoring HP as the weapon of choice. You can accomplish this via a main attack, or as a feature (such as an aura, or bonus action that can inflict conditions).
Stats are a bit more malleable, depending on what CR you are shooting for. But it would have to have a base speed of 40 (in order to not be outpaced), with some form of advanced movement. Preferably tunneling, though swimming and flight work well enough.
Either way, they are going to need a ranged attack to counter anyone who tries to get clever with flying. The good old Rock attack works splendidly for this, as it can also knock flying targets prone.
An advanced sense will also help. If you have Tunneling, Tremorsense is a must, but you should have a backup to prevent the monster from becoming too gimmicky. Truesight is obviously the best, if you can justify it.
Defenses can also have a massive impact on how strong a monster is. If something has any form of regeneration, and a way to run away and hide, it can massively up it's threat by using hit and run tactics. Unfortunately, this method can be a bit too extreme if the PCs have no way of preventing the creature from running way. Making it better to have other kinds of defensive options. A Reaction that can block attacks or counter spells is one of the better ways to accomplish this. A novel concept would be some form of ablative defense that can remove conditions or absorb some damage.
Given all that, I would make a "Heat Crawler"
A giant sickly glowing green centipede-like creature that burrows around near the surface, looking for food with it's tremorsense. Once it finds a target, it erupts from the ground, grapples a target, and drags them below the ground. Where the strange energies emitted from its body cook the victim alive, exhausting the prey in the process. Should the prey be flighty, they can use their serpentine body to toss a rock, knocking the prey prone, so they can give chase. The Heat Crawler's carapace gives them resistance to damage, but they can shed it (requiring no action) in order to instantly end any status condition or ongoing effect that requires a saving throw, and doing so removes their damage reduction until they complete a long rest.
This was my answer. The old school kind, not so much the 5e version.Nilbog.
When I took over DMing for my group (back in the 2E days) after a rather overpowered adventure by our other DM, one of the players had a pet baby gold dragon. I decided that the reason why dragons hoard gold is that they need it to grow and mature. So this pet dragon started eating all the party's gold - and then demanding more. And it got bigger and grumpier the more it ate.The Gas Spore is entirely anti-player meta.
My favorite from 1E was the Goldbug. It didn't kill the character, but drove the player bat-crap crazy. It was basically indistinguishable from a gold piece, and it ate precious metals. PCs take them as treasure, put them in their stash, then find it smaller each time they check on it.