D&D 5E Best low level monsters to freak the hell out of a low-mid level party?

[Erechel]

I'm being mastering D&D 5º since it came out, and I have directed two different tables, and I find ot several things:

• Because of the economy of rounds and battles, not many single piñata monsters, like let's say an ogre, can pose a threat to a party, because of the amount of damage that a single player can output. And low-initiative monsters are usually cannon fodder. Massive damage can put a player in death risk, but it won't necessarily won a battle, because his allies usually kill it before the downed character has made the three Death Saves.
• Creatures with damage resistance and stealth abilities are usually very terryfying.
• Creatures that drain stats are the most terryfying foes, because they thwart the ability of players to fight them.
• Undead have no care for conditions that other creatures have (suffocation -they can go underwater easily-, fear, poison, disease).

I reach to the conclussion that Shadows are one of the most dangerous, obnoxious and hated creatures. Orcs with chainmail and shield are terrific, as they can think a strategy and coordinated action, but ultimately they won't last long; Zombies are cool, because Undead Fortitude, but they lack the frightening factor of a "converting" bite, and they act last. But shadows... ten shadows can destroy the morale of the most tenacious group, as they have resistance to almost everything, they drain strenght (a fact that even non strenght-based character fear, as it conducts to instant death, but the paladin or the fighter will hate too), they can fastly reproduce, they have an almost unsurpassable stealth factor, and they are as scary as . My groups hate them, fear them, and will run of them as far as they encounter them. What monsters do you think that are scariest to players and characters?

 

[iserith]

Rust monsters.

 

[SilverfireSage]

An illusionist. Playing with players minds and expectations can be more fun than actually throwing the real monster at them.

"He would never throw a dragon that big at us!"
"Would he?"

Barring that, oozes are fun when characters start to get trapped in them, ghosts are terrifying just on their own... gosh, really anything can do a good job depending on how you run it.

 

[The Grassy Gnoll]

My guys HATE orcs. Purely due to bad dice rolls, their first encounter with orcs was a 4-Orc raiding patrol that VERY nearly TPK'd the party and they haven't forgotten that. I of course now like to put orcs everywhere I don't want the players to go...
*evil laugh*
*strokes white cat*
*twirls moustache*

 

[Tormyr]

Intellect Devourers

 

[Erechel]

People, give some intel. I'm stealing ideas here

 

[Cadriel]

One of my favorites was the Will o' the Wisp. Between the death ability, the shocking ability, and its extensive damage resistances, it was easily the most intense 5e encounter I've run. Magic Missile came in handy, but it was very touch and go for a few rounds.

Things with DR to the casters' damage cantrips can be quite nasty. Teleporting is also a lot of fun.

 

[Greenstone.Walker]

Tucker's Kobolds.

 

[ExploderWizard]

I brought these special guests in from B/X to terrorize my players:

Thoul
Medium monstrosity , chaotic evil
Armor Class 15 (natural armor)
Hit Points 62 (8d8+16 )
Speed 30 ft.
STR DEX CON INT WIS CHA
16 (+3) 15 (+2) 15 (+2) 10(0) 14 (+2) 8 (−1)
Damage Immunities poison
Condition Immunities charmed, exhaustion, poisoned
Senses darkvision 60 ft., passive Perception 10
Languages Common
Challenge 4 (1100 XP)
Keen Smell. The thoul has advantage on Wisdom (Perception) checks that rely on smell.

Regeneration. The thoul regains 10 hit points at the start of its turn. If the thoul takes acid or fire damage, this trait doesn’t function at the start of the thoul’s next turn. The thoul dies only if it starts its turn with 0 hit points and doesn’t regenerate.

Martial Advantage. Once per turn, the thoul can deal an extra (2d6) damage to a creature it hits with a claw or bite attack if that creature is within 5 feet of an ally of the thoul that isn’t incapacitated.

Actions
Multiattack. The thoul makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: (2d6 + 3) piercing damage.

Claws. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: (2d4 + 3) slashing damage. If the target is a creature other than an elf or undead, it must succeed on a DC 10 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success

 

[Sunseeker]

One of my favorites was the Will o' the Wisp. Between the death ability, the shocking ability, and its extensive damage resistances, it was easily the most intense 5e encounter I've run. Magic Missile came in handy, but it was very touch and go for a few rounds.

Things with DR to the casters' damage cantrips can be quite nasty. Teleporting is also a lot of fun.

I have to agree with the Will 'o the Wisp on this. They're fairly unassuming creatures which makes them so darn awesome because people are expecting something simple, like a ghost but dumber, then it nearly eats them alive, summons friends and tries to drown the party. A lot of people assume that if you avoid going near one, they won't bother you, as their intention is merely to lure you to your demise (because its assumed they're incapable of doing so themselves), but they are quite capable and aren't just floating lanterns in the distance, they will actively try to mess you up.