* {{Altitis}}: The extensive character customization and the number of paths you can take tends to encourage this in players. Equally encouraged is reincarnating a singular character, which maintains certain attributes (tomes, epic destinies, reaper XP) while adding past life bonuses and allowing the same experimentation of different classes.
* CompleteMonster
** ("Demon Sands" module): [[GodSaveUsFromTheQueen Queen Lailat]] is the [[OurDemonsAreDifferent demonic]] ruler of the Menechtarun desert and the one behind the region's perpetual violence. Summoned to Eberron by King Raiyum, Lailat helped him achieve lichdom and then imprisoned him in his tomb so that she could turn his kingdom into her own personal killing field. Lailat led the Firebrand gnolls to become vicious man-eating slavers, taught the Vulkoorim scorrow to adopt [[HumanSacrifice ritual sacrifice]], and set them against each other and Menechtarun's surviving humans simply to enjoy the violence. When the players raid her lair, Lailat is shown to have killed another adventuring party and revived them to fight for her as undead, [[AndIMustScream reanimating them each time they die to ensure that their suffering never ends]]. She also keeps a rakshasa bound as a SexSlave and considers replacing him with a player when he dies. A boundlessly cruel tyrant concerned only with causing as much violence and suffering as she can, Lailat is a vicious monster even among demons.
** {"City of Sharn" module): Lucian Vaunt is a relatively [[FromNobodyToNightmare ordinary man]] with no special skills or abilities, besides being an intelligent entrepreneur trying to make a name for himself in the city of Sharn. He discovers a way to bring magic to the masses by inventing a device that can allow ordinary people to cast spells, and gets rich building a business empire around his product. Unfortunately for his customers, his devices [[CastFromLifespan drain their life energy]] to work which slowly kills them off. Also, the necromancers he employed to design and perfect his devices are the only ones who [[LeaveNoWitnesses know of this]], and they all suddenly die [[MakeItLookLikeAnAccident in a horrible freak accidental explosion]] in the industrial sector of Sharn, but not before casting a curse against Vaunt that will [[MurderIntoMalevolence cause anyone to die in relation to what Vaunt did]] to be resurrected as a Forgewraith - a [[VengefulGhost ghostly undead flaming revenant]] with the goal of murdering Vaunt and anyone else still living in Sharn. Finally to combat the Forgewraith problem, Vaunt steals the Stormreach beacon - a magical artifact that will [[PowerNullifier paralyse the Forgewraiths]], but is also the only thing holding back an ancient madness curse that will turn all of Stormreach into a bloodbath of insanity. Vaunt does all of these horrible deeds which kills hundreds if not thousands directly or indirectly, and does not care one bit about any of it. His only concern is to gain as much wealth and power as possible amongst the elites of Sharn.
* DemonicSpiders:
** Kobold Shamans on elite mode know the Lightning Bolt spell and love to spam it. They are far more dangerous than their hard mode counterparts and pretty much anything else encountered alongside them (except perhaps wolves).
** Beholders are especially dangerous because of their anti-magic field and deadly spells. Just being within sight of one probably means you will lose all your magical buffs and become unable to cast spells, all while they fling Disintegrate spells at you.
** [[EliteMook Monster Champions,]] monsters with crowns over their heads that deal significantly more damage, have far more HP, and gain extra benefits like Deathblock and damage reduction on top of that.
** Some Elite monsters cause extra damage or add a damage-over-time effect every time they damage you. Combined with spells or abilities that cause damage multiple times at once such as Magic Missile, or monsters that attack very quickly, this can very quickly kill an insufficiently-protected player.
** Taken up to eleven with Reapers, which can spawn in any quest that's on Reaper difficulty, hit even harder than the aforementioned Champions, have 50% incorporeality, and slow down their targets when they attack. Try to range them from far away, or from a high ledge they can't move to normally? They'll just warp right up to you.
* FandomRivalry: A somewhat {{downplayed|trope}} example -- being two [=MMORPGs=] based on the same franchise, this was bound to happen between ''VideoGame/{{Neverwinter}}'' and ''VideoGame/DungeonsAndDragonsOnline'' players. The rivalry come more from the fact that DDO uses the way-less popular ''Eberron'' setting while ''VideoGame/{{Neverwinter}}'' uses the more mainstream ''Forgotten Realms'' setting (DDO eventually got the ''Forgotten Realms'' setting in an expansion, funnily enough) and the obligatory discussions about what game respects the spirit of their namesake Tabletop Game more. The general consensus is that Neverwinter is the easier game to learn and play casually, but feels more like any other generic fantasy game, while DDO is more faithful to the tabletop rules and feels more like the pen and paper game brought to the videogame world.
* GoddamnBats: As in any game with this wide an array of monsters, these are common and varied.
** Actual bats are extremely twitchy and hard to land a swing on without using the slower, less efficient auto-target or an [=AoE=] attack because of their habit of constantly circling a player. Spiders, to a lesser extent, love to leap past you trying to attack.
** Rust monsters and oozes for melee fighters since both severely damage metal equipment that hits them, which is almost everything melee fighters use. Wooden weapons (for rust monsters only), Handwraps, Muckbane, Muck's Doom, or Everbright weapons are the only things that avoid this.
** Any monster that can CC the player, because no matter how high your saves are you can always roll a one. There are a lot of these, including spellcasters who cast Daze, Hold, Hypnotize, (Ir)Resistible Dance, or Soundburst, monsters with (Improved) Trip, and things like Grease or Sleet Storm that create slippery ground and remove the ability to jump.
** Archer enemies have higher than average hitpoints, lower than average damage, and like to pop up in places melee fighters can't easily get to them.
* SugarWiki/HeartwarmingMoments: Creator/GaryGygax, co-creator of the original ''TabletopGame/DungeonsAndDragons'' game, actually narrated one quest arc, Delera's Tomb. After Gygax's death in 2008, the developers of DDO created an in-game memorial in Delera's graveyard (which includes a twenty-sided die on the stone memorial). A similar memorial appears in the Threnal quest chain for D&D co-creator Dave Arneson, who narrated that quest series.
* NightmareFuel: The Taken, a game-original aberration race, are probably the most disturbing monsters in the game. Not because of their appearance, which is [[BodyHorror bad enough]] but because they are made by melting down and reshaping ''people''. Some distorted human features can be seen on their monstrous bodies, and there are hints that the component people are [[AndIMustScream still aware in some way]]. Even worse, the mind flayer who made them is confirmed to have used [[WouldHurtAChild kids]].
* ScrappyMechanic:
** Poison has become this after Menace of the Underdark. It used to function much like it does in D&D 3.5 and was not a serious threat to a veteran adventurer, much like on the tabletop. However, with this expansion and its focus on drow and poison, they redesigned the Poison system completely so Poison can also deal hit point damage as well as ability damage and there are multiple poison types with various conditions and protections and different saving throws for each type based on what type it is. This has led to a great deal of frustration and confusion over how Poison actually works and what protects you from what, since there's no easy way to tell anymore.
** Taking any amount of damage will interrupt any action, such as throwing a switch or opening a door. This can lead to times where you have to kill everything in the room just so you don't get interrupted by constant plinks for trivial amounts of damage.
* ThatOneBoss:
** Gnarkill, Troll Butcher in the Waterworks area is the first troll parties are likely to encounter. His regeneration combined with heavy damage-dealing capabilities make him difficult for parties that do not know to use fire or acid. Considering that flaming and acid weapons are not common at low levels (Perceptive people might notice [[BossArenaIdiocy the very helpful flame trap with a pressure pad on the right side of the room]])...
** First General Horoth in the Tower of Despair raid has around 2 million health, hits hard, and regains 20% of his health if a party member dies. He also has a move that will literally BANISH those not wearing Boots of Anchoring,them back to Stormreach, denying them completion of the raid and the resulting quest loot and will stun those who are for a short time.
** Also in the Tower of Despair, the Shadowmaster is not a very strong foe by himself, but has minions that attack during the fight and deal around 300 cold damage just by being near you. If they are not kited properly, they will quickly wipe the raid.
** The Black Abbot, raid boss of the Ascension Chamber, is a powerful spellcaster with 400-damage disintegrates, a move that sets the arena on fire and drains the casters' mana, a move that traps one person, deals damage, and drains their mana, and can summon invisible enemies that, you guessed it, drain mana. In addition, the battle takes place on a platform floating on a lake of death. Oh, and you need to complete puzzles to finish him off.
** The Lord of Blades. He is a handful on the lowest available difficulty setting and {{harder than hard}} on Epic. On Normal, he has ~200k hitpoints (about as much as several raid bosses have on Epic), 80% fortification with a such a massive fortitude save that he is pretty much sunder proof, has several devastating AOE attacks which are capable of instantly killing almost any character if you do not evade them (requiring twitch skills), and the fight goes in several phases which involve periodically clearing minibosses while still enduring LOB's powerful attacks.
*** On Normal, his Challenge Rating is the highest in the game at 32. This means he is seven levels higher than the current level cap (twelve higher than the previous one). On Epic, his CR is '''46'''.
*** Also, due to the new expansion, Epic mode has been divided into four difficulties.(Epic Casual, Normal, Hard, and Elite) Epic Hard is usually much easier than the old epic, but [[SerialEscalation Epic Elite is even harder than the old Epic difficulty, meaning that the Lord of Blades is even tougher than before.]]
* ThatOneLevel:
** "Proof is in the Poison" is very much a nightmare for any low-level party that does not know what they're doing, mainly due to the many casters in the dungeon that like to throw nasty spells on you. The Quickfoot Caster ambush on the bridge at the beginning has led to many a TotalPartyKill. Also, there are 3 rooms almost completely filled with acid, and you either need to navigate across pipes on the ceiling or jump on small platforms to the other side to pull a lever, causing walls to go up in the pool, making it so you need to return a certain way. Also, there's a room with acid in front of the staircases, as well as an earlier room where you will likely need to walk across a pool of acid. As if that's not bad enough, the third pool of acid has spinning blades in it.
** "Kobold Assault" gets this for being the first Challenge Dungeon most players meet, It's a massive ZergRush of 200 Kobolds, including their spellcasters, and the singular rest shrine is out in the open.
** "In The Demon's Den" on elite difficulty. A base level 18 quest, you must kite an invincible [[MultiArmedAndDangerous six-armed]] 30-CR red name boss while wearing down and simultaneously killing '''three''' other 29-CR red name bosses just to remove the first one's invincibility. Even worse, dispels are going to hit you left and right, so spells like Haste and Greater Heroism can't help you. Nowadays, six level 20's with strong builds and maxed epic destinies can expect to breeze right through. Back when it was first released[[note]]in 2010; epic destinies were added mid-2012[[/note]], or with weaker groups who are expecting difficulty comparable to other level 18's, this quest will ''murder'' you.
** The entire Threnal adventure pack, not for its difficulty but for its hair-pulling frustration - It blends GuideDangIt, EscortMission, and GoddamnedBats in equal proportions on top of being incredibly long and not particularly worthwhile except for the end rewards. What makes this quest chain so bad? For starters it's ''extremely'' long, having three quest arcs of three quests each to finish it, and those quests are not short (except for, ironically, [[DisappointingLastLevel the finale]]), not worth a lot of experience, and don't have great loot apart from the reward for finishing the whole nine-quest arc. Of those nine quests, three of them are [[EscortMission escort quests]] where you have to lead [=NPCs=] around or protect them, or both. Two more of those quests involve hunting down keys in a winding network of tunnels unless you have a lockpicker, and one of them requires you to [[GuideDangIt shoot a target lever at the very end]] when you're most likely to have run out of ammunition (and unlike in quests produced or retouched later, none is provided). Assuming any of you were carrying a ranged weapon in the first place. And if you're solo, it's possible the quest might reset while you're leaving to get one, making you do the whole thing over. The quests are a melee fighter's nightmare on top of all that, featuring monsters with high damage reduction you probably can't overcome at the recommended level as well as tons of weapon-destroying oozes and rust monsters in addition to aforementioned target lever. Later on you'll begin to find beholders, including a CR 17 elder beholder (in a level 9 quest on Normal!), which can nullify all your buffs or even kill you instantly. If that weren't enough, it's also possible to bug out the quest-givers, forcing you to repeat parts you've already done unless you speak to the right people in the right order.