The Dungeons & Dragons Monster Manual is packed with creatures from the tiniest spider to the unconquerable tarrasque. There's a lot of variety to choose from when building a challenging adventure. Ideally, your party should be able to handle anything the DM throws at them, at least theoretically.
Sometimes, however, the party might underestimate a powerful monster. Not everything capable of a total party kill is a five story tall monstrosity. We're looking at some of the scariest monsters with less-than-scary packages.
10 Wolves
We're starting the list off with the humble wolf. "Wolves?" You might ask. "Like a real thing that exists in real life?" Listen, could you fight off a wolf? Let alone a whole pack of wolves. Adventurers don't come into the world with the power of a god (usually). Sometimes they're just folks that are good at swinging a sword, and wolves can be very dangerous foes.
The thing about wolves is that you will almost always encounter them in packs. "The lone wolf dies" and all that. And wolves also have a feature called Pack Tactics, which gives them advantage (rolling two d20s and taking the highest) on attacks as long as one of their allies is nearby. A low level party can get absolutely savaged by a pack of wolves if the dice are against them.
9 Gas Spore
Does fungus count as a monster? Well, it's in the Monster Manual, so why not! A gas spore is literally just a floating ball of fungus filled with horribly toxic gas. The first spores were thought to have emerged from the bodies of dead beholders. They also resemble beholders: spheres floating through dungeons, complete with a false "eye" and growths on the top that look like eye stalks. This weird ball of gas isn't going to attack you; the real danger comes from you attacking it. If the gasbag is pierced, it releases spores that infect anyone who breathes them, and the spores kill you dead within a day unless the disease is magically removed.
8 Doppleganger
Looking like little more than weird grey-skinned humanoids, doppelgangers have the ability to take on the form of anyone it's seen before. This makes them adept at infiltrating parties posing as a beloved comrade—who might even now be lying dead somewhere behind them. The "destroy a party" might be more figurative than literal in this sense. A group of adventurers with a few levels under their belt would probably be able to take a doppelganger in a fair fight, but they're unlikely to get the chance. Doppelgangers want to deceive you. They can introduce strife into a group, lead party members into traps, and try to tear them apart from the inside.
7 Will-o'-Wisp
These tiny undead spirits definitely seem trivial at first. Almost completely incorporeal balls of light; is it going to twinkle you to death? So it might come as something of a shock when it zaps you with lightning magic. But even that's not what makes the will-o'-wisp truly dangerous. It's that they tend to inhabit dark and dangerous places, and from a distance these friendly flickering lights might look like civilization.
Unfortunately, you're much more likely to follow them into the lair of a very dangerous creature like a necromancer or a black dragon. And while you're busy fighting the larger threat, the will-o'-wisp patiently waits for you to drop to 0 hit points. When you do, the wisp swoops in. It'll try to steal your life force—fail a save and you're dead. No takesies-backsies.
6 Intellect Devourer
This creature is classed as "tiny". It's really just a brain with paws. It looks like something you could punt into next week. It isn't much of a threat in a one-on-one fight, but it does have the disturbing ability to "devour intellect" as the name suggests. It can target any creature nearby that has a brain (so barbarians are generally safe) and force an Intelligence contest. If the character loses, they take psychic damage and have the potential to have their Intelligence score reduced to 0, rendering them incapacitated until they can somehow regain those points. Plus, if your comatose body loses another Intelligence contest with the creature, it eats and replaces your brain!
5 Hag
Often seeming at first glance to just be an old woman, hags are some of the most dangerous things a lower-level party can encounter. They tend to be at the root of foul happenings or dark bargains, the kind of thing adventurers are usually brought in to solve. There are a few different types of hag of varying levels of danger, but they're all competent spellcasters and they tend to have other dangerous abilities as well. They can turn invisible, cause sickening dreams, or even—in the case of a sea hag—shoot you a nasty enough look to instantly drop you to 0 hit points. Plus sometimes they form covens! Buy one, get two free.
4 Stirge
Okay, now surely this is going too far. Stirges only have two hit points. What could this horrible bat...rat...mosquito do to a party of big strong adventurers? Well, even adventurers need blood, as the saying goes. The dangerous thing is about stirges is that once they're attached they can keep sucking your blood either until they're full or you die. They don't have to make an attack roll once they've got their disgusting snout in you. You can pull them off, but only one at a time. If you have the misfortune of running into a whole cloud of stirges, they could put down an unlucky low level party in no time flat.
3 Black Pudding
One of the subcategories of ooze, a black pudding is a sticky black sludge that creeps through dark dungeons, slurping up anything in its path. And apparently in those low light conditions, the pudding can appear like little more than a shadow, making it easy for the unwary to literally stumble into it. Just touching it causes material other than stone to corrode and dissolve. That doesn't just include flesh, but also your armor and weapons. Plus, if the pudding takes any lightning or slashing damage, it can split into two new smaller puddings for double the trouble.
2 Mimic
Mimics are probably the quintessential example of "looks safe but is actually deadly." At rest, they just look like chests or doors. Something a curious adventurer might touch. It secretes an adhesive substance to trap anything that touches it, and will then proceed to bite and batter it into submission. One mimic probably isn't enough to wipe out a well-rested adventurer, but if you're cracking open a chest hoping for a health potion, you might be in for a nasty surprise.
1 Demilich
By far the most dangerous creature on this list, a demilich is also probably one of the most innocuous. You're bound to see plenty of human remains while plundering tombs and dungeons. A demilich is just a skull, all that remains of a true lich that has failed to keep its phylactery fed with souls. It's only when the skull's lair is disturbed that it reveals its true nature.
While it doesn't have the magical prowess it had as a lich, it still has the power to drain vitality from the living. It's most terrifying ability, however, is its ghastly howl. Just hearing it can drop people to 0 hit points, and any who survive are likely to run screaming for the hills.