D&D 5E Most deadly monster?

[J.Quondam]

• Manticore. This CR3 doesn't appear crazy until you have it fly from afar and pepper any ranged PCs with tail spike attacks till they die. Any monster that can fly and (effectively) attack from afar fits this category, such as bow-equipped aarakokra.

Manticore is a great, especially since it's likely the very first monster encountered in the Essentials kit -- a tiny, little, TPK-ready quest, for a reward of 25 shiny gold pieces. Great stuff for a brand new DM with brand new players!

 

[aco175]

Mosquitoes, those things carry malaria. Most people only have a Con of 10.

 

[Nefermandias]

Shadows can easily delete any pesky PC with their strength draining touch.

 

[Ralif Redhammer]

If they don't manage to do enough to disrupt the ritual, yeah, the PCs are walking into a death trap with that encounter. My party managed to do enough to make her manageable, but even though they were about 12-strong, she still held her own against them and the only reason no one died was pretty much because the cleric spent every turn healing.

Tyranny of Dragons' Tiamat.

 

[Krachek]

Any monster that attacks down PC for fun.

 

[CleverNickName]

Any monster that attacks down PC for fun.

As far as I can tell, that covers 3 categories of monsters.
(a) the very cruel and evil,
(b) the very cunning and intelligent, and
(c) the very hungry and non-intelligent.

 

[cbwjm]

Mantis shrimp, the tiny killer.

 

[Mort]

I wouldn't call them deadly, but the monster that elicited the most abject terror from the players - the rust monster.

Group had just discovered a cache of Adamantine weapons and armor. The look of fear on the PCs faces when a bunch of Rust monsters showed up was absolutely priceless.

This cache wasn't magical, but if it ever comes up again with magical loot - I'm definitely ignoring the 5e Rust Monster caveat that magic weapons armor are unaffected!

This goes with something I've found over the years. Attack the characters - even with overwhelming lethal force? Most players react with meh, that's what happens in the game.

Attack/steal/take away their stuff? Most players react with near irrational levels of fear/anger/resentment against the perpetrating party!

 

[DND_Reborn]

Just thought of another one: Carrion Crawlers

Especially deadly in prior editions, but still quite nasty in 5E IMO.

 

[Mort]

Properly played, Helmed Horrors can be pretty nasty - especially with the proper spell immunities and if the DM is stingy with magical weapons.