The continent of Faerun is brimming with threats both human, animal, and monsters for adventurers to skill or talk out of combat with. It just so happens that around many of them also tend to kill the adventurers first-- some can even cause party wipes in the most hilarious and hopeless of ways.
This is great for the Dungeon Master who gets to be entertained at the sight of hapless and boastful players underestimating their enemies. Other times, these threatening D&D creatures can put power-players and min-maxers in their place as those guys tend to ruin the fun for everyone. Hence, here are 10 dangerous monsters for DMs to consider and for players to pray they don't encounter.
10 A BUNCH OF GHOULS
Sure one, two, or three ghouls shouldn't really pose any kind of threats for tier 1 or even tier 2 players, but a handful or several dozen of them? It might as well be a slow death sentence. Each ghoul has a 50/50 chance of paralyzing any player they claw and when there are several dozens of them attacking, there goes interactivity.
In fact, around two or three ghouls for each player should be enough to cause a party wipe especially if they didn't bring a cleric or a paladin. During most of the fight, they'd either be rolling for paralysis checks or just skipping their turns entirely. It's a funny kind of party wipe though.
9 RED DRAGONS
Whether its a young red dragon or an adult one, these flying overgrown lizards are the bane of battlefields. Their most dangerous attack, the cone of fire, can wipe out anyone whole parties without ample fire resistance and even trouble those that do.
When not punishing players for clumping together like horror house visitors, these red dragons will be modifying the battlefield makeup with their Frightful Presence action, essentially making everyone run. Forget about using stunning or cursing it, the adult version has legendary resistance.
8 MIND FLAYER
As a common courtesy for t1 or t2 players, DMs tend to minimize the number of Mind Flayers per encounter to just 2 or 3. Because more than that and most adventurers would just be nursing headaches (pun intended) and getting their brains extracted the whole fight.
Mind Flayers, like dragons, have a cone AoE attack but psychic instead of fire; anyone who fails the DC gets stunned for a minute. Trying to fight four or five of these guys at the same time is just begging to get stun-locked.
7 HAGS
There are many kinds of hags in D&D and not many of them are really that threatening in combat once players get past tier 1. However, where they excel in ruining adventurers' lives is how they can change the narrative in their favor.
Hags can pretty much shapeshift or make illusions to ruin the whole party's direction or even lure them into traps. These traps can then be more dangerous than the hags themselves. Some of them can even pose as NPCs; depending on how the DM gets creative with them, they can cause an unintended party wipe.
6 GAS SPORES
Gas Spores are the discount beholders of D&D and they're a great way of introducing the party to beholder-like creatures at low levels, or in tier 1. They look threatening enough to be poked by swords or arrows, except they don't actually pose any dangers if unattacked.
Any party of player foolish enough to gang up on a gas spore, however, will likely find themselves dead or clinging to dear life within a day. These things explode into spores when pierced and will infect the players unless the spores are magically removed. It's a gift that keeps on giving.
5 DEMILICH
The Demilich is like a Mind Flayer except they have legendary resistance and can easily shrug off debilitating status effects. They do have low HP but they make up for it with their resistance or immunity to everything, including turn undead spells.
Moreover, they can easily frighten opponents and change the battlefield to their favor. While they don't have attacks as powerful as a dragon's or a Mind Flayer's their curses and that fact that they're usually accompanied by other undead-- such as ghouls, make them scary boss material.
4 BLACK PUDDING
Want to annoy players and give them a nemesis enemy that keeps coming back and escaping when near dead? Give them the black pudding. It's an ooze-type creature that can fit and squeeze in anywhere and is immune to all status effects.
The black pudding also destroys or corrodes any nonmagical weapon it touches. When players do get to hit it successfully, it will split into two and the players will have to deal with those again. For t1 players, this thing is a nightmare to face.
3 HECATONCHIERE
Speaking of nightmares, Hecatonchieres used to be the ultimate giants back in 3.5e of D&D. Thankfully, 5e doesn't have them officially but any brutal or unforgiving homebrew can use them. They're a good way to traumatize players.
These things are abomination giants that are hundred-foot-tall amalgams of 50 angry people. They have 100 arms and thus can attack a single player 10 times each round. It practically kills one player each round. That's a rather short four to six-turn survival clock.
2 BEHOLDER
Not many players can claim they have successfully defeated a beholder; those that do probably have pretty bad or benevolent DMs. Because beholders can change reality at their whim.
Each beholder also has multiple tentacles that each come with a debilitating status effect. The beholder can attack with three or four rays each of varying effect every turn and this can incapacitate the whole party in just one turn. If written well and imaginatively, they beholders can be reality-bending boss monsters.
1 TARRASQUE
Speaking of boss monsters, no other creature in the monster compendium of 5e can challenge the Tarrasque when it comes to HP. It has the highest HP of all monsters (600+), makes five or six attacks each turn, and can swallow players whole per round.
It's also got Frightful Presence in addition to having legendary resistance. That's only some of the few reasons why the Tarrasque is usually reserved for level 20 or t4 players in D&D. It can eat the whole party in two or three turns and use their bones as toothpicks; if it doesn't then the DM is boring.