Dungeons & Dragons is a tabletop RPG with a strong emphasis on combat, complete with monster stat blocks, figurines, maps, and deadly spells of all kinds. Experienced players are wary of deadly monsters like death knights and ancient dragons in combat, but some creatures are actually stronger during roleplay sessions rather than in battle.
Part of the fun of D&D is having the characters engage with bizarre and dangerous creatures with clever roleplay-based tactics rather than straightforward combat. Those creatures do have stat blocks for proper combat, but if the DM handles them correctly, they are scarier, cooler, and more intriguing in between battles. Some of these monsters might be roleplay-only, while other non-combat encounters with them might smoothly transition into a proper battle once the initial shock wears off.
10 Doppelgangers
Doppelgangers are exactly what novice D&D players will expect them to be: shapeshifting humanoids and masters of disguise and deception. Doppelgangers are not evil or always malicious, but they will still keep the party on their toes since they can be anyone, anywhere, and strike without warning.
Doppelgangers are weak and boring in combat, so they are best used during roleplay. Ideally, the DM will keep the party guessing about who's who, but gradually reveal hints so the party can have fun deducing the doppelganger's identity and cornering it at last.
9 Slaad Tadpoles
Slaad tadpoles rank among D&D's weakest creatures in combat, and even level 1 players can easily kill them. Instead, the DM should use slaad tadpoles to add gory horror elements to the game, and the stat blocks of adult slaad creatures such as the red slaad make it clear how that should work.
Slaads will implant their eggs into a victim, and then the slaad tadpole will hatch and eat its way out of the host. A DM can shock and horrify their players with a sudden slaad tadpole bursting from an NPC's chest like a scene straight out of 1979's Alien. An infestation of slaad tadpoles may not be a threat in itself, but it hints at the presence of some much more powerful creatures to come.
8 Rust Monsters
Rust monsters aren't very powerful in combat, having low hit points and a weak bite attack that even low-level parties won't fear. What makes rust monsters so dangerous is their trademark ability to corrode metal objects and turn them to worthless rust.
Rust monsters can do this in combat too, but they're more effective in roleplay sequences. For example, these insidious creatures might sneak into the party's camp to ruin all their equipment, or they could wreak havoc in a weapons and armor store, obliterating the inventory in a single night.
7 Medusas
The medusa is an iconic monster from Greek myth, a humanoid female famous for its snake hair and its petrifying gaze. In D&D, medusas are a cool and challenging monster to fight in actual combat, but a clever DM can make more use of medusas than that.
A low-level party may be asked to sneak in or out of a location while a medusa is on the prowl, and meeting that creature's gaze means game over. This could make for a tense, terrifying, and memorable sequence where the players must dodge the monster's gaze, not hack away at its hit points.
6 Succubi/Incubi
Succubi and their male counterparts, incubi, are moderately dangerous in combat, having a variety of damage resistances and a Draining Kiss attack that deals ample psychic damage. Given a succubus' abilities and lore, though, these creatures are more fun in roleplaying sessions.
A succubus or incubus can use telepathy and shapeshifting to stalk and deceive a player character, or they might go on a rampage in a town, challenging the party to track down the elusive monster. The final fight might even be between the party members and their own mind-controlled allies rather than the succubus itself.
5 Meenlocks
Meenlocks are small, evil fey creatures that are only challenging to low-level parties if a combat encounter involves an overwhelming number of them. Instead, meenlocks are scarier in roleplaying, and they even feature in an eerie mystery adventure in the Candlekeep Mysteries book.
Meenlocks are cruel psychic fey who use mental tricks to terrify and upset their targets, eroding their mental state until the victim is dragged back to the meenlocks' lair. These creatures may fool the party into thinking the threat is something even nastier, scaring away a party that would normally fight off meenlocks with ease.
4 Purple Worms
Purple worms are D&D's version of the great sandworms from Dune lore, giant armored creatures who burrow deep in the desert and sometimes pop up to eat whatever fits into their mouths. Purple worms are tough in a fight, but in a desert adventure, they make excellent environmental threats as well.
A medium-level party will probably avoid combat with purple worms if they can. The DM can send a purple worm after the party, challenging the party to flee and reach safety before they all get swallowed. In some cases, though, a clever and gutsy druid or ranger might try to ride a purple worm like Paul Atreides, just for fun.
3 Elder Brains
Elder Brains are tough in a fight, but these ultra-intelligent creatures would rather avoid direct combat and manipulate people from afar with their psychic powers. An elder brain can boss around its mind flayer minions to wreak havoc in a kingdom, and the brain will see all and know all.
All this should make for some exciting but scary gameplay where the party simply cannot outwit the elder brain or find a way into its lair. Barbarians and fighters will be especially frustrated, since they can hack apart the elder brain's thralls but cannot fight the elder brain directly in combat.
2 Slithering Trackers
Slithering trackers are evil oozes that only have modest stats for combat, being CR 3 to threaten low-level parties. They're fairly straightforward in battle as slippery creatures that can grapple a target and deal necrotic damge, but in roleplay, they are far scarier.
A slithering tracker can easily stalk the party or ambush them by squeezing through tight spaces and hiding in water. It can then attack either a party member or an NPC, suffocating them with its ooze grip and strangling them if the party can't get it off in time.
1 Vampires
Vampires are powerful and flexible in combat, so they can be a fun mini-boss for campaigns inspired by the classic Curse of Strahd. Vampires can do all kinds of things in proper combat, but these famous creatures are more exciting and intriguing during roleplay sessions instead.
A vampire may stalk or offer a bargain to a low-level party whose members can't easily fight off this monster. The party will know they're in extreme danger, but if they roleplay carefully with a charisma-heavy character or flee, they just might escape the vampire's wrath. But if they fail, death is certain, and that cranks up the tension and suspense.