Dungeons are full of mob fights, and it's the primary place to get Amberstone and Orbs, which can be dropped by any mob in the dungeon.
Orbs - Each dungeon offers a unique orb drop to their dungeon.
Gates (Valnir) - Void orb
Darkness - Orb of Shadows
Hurricane - Gale Orb
Labyrinth - Orb of Drought
Hell - Solar Orb
Gates Dungeon A timed boss event battle, unlocked with Scavenger rank.
Valnir spawns there every so many hours, usually 5.
He's currently in the form of a super powered wither skeleton, and can one-shot-kill most wannabe heroes. Spawns TNT, causes blindness/confusion, spawns cobwebs, among other tricks. Not to be faced alone.
The times in Eastern time zone are 12am, 5am, 10am, 3pm, 8pm.
There are no other mobs to fight here, it's a set event battle.
Darkness Dungeon - Very dark, night vision recommended. Unlocked with the Explorer rank.
Wanderer look like skeletons, but are actually a melee mob. Dangerous in swarms.
Living Bomb look like creepers. They do explode, but seem to do no damage to you. They will hurt other mobs though.
The Lost A super powered enderman, this guy hangs out in the back, right hand side of the dungeon. He hits hard, and most prefer help to take him on. Arrows are useless, but potions might help you. Most agree that this is the hardest boss in all the dungeons.
Summer Boss 2019 - A superpowered wither skeleton, he's like the Valnir without the tricks. Quite fast and powerful, you're best off to avoid him unless you have to fight him. He hangs out near the center of the dungeon, usually towards the back half.
Hurricane Dungeon - Cold and mountainous, be prepared to take fall damage, and watch your armor durability at all times. Unlocked with the Paladin rank.
Guilt these are based off the Stray, the snow biome skeleton. As such they drop arrows of slowness when killed. Not overly dangerous on their own, but dangerous in swarms, especially when combined with the attacks of their little flying buddies.
Karma these are Vex, and are the bane of this dungeon.They will swarm you, hit you from every angle, and do a number on your armor. Not recommended to fight these if you're having lag issues.
Regret these are Ghasts, and good for farming tears - if you can kill them with the other mobs swarming you that is. Strangely enough, it seems their fireballs don't hurt you, but will block your arrows completely, so time your shots. Their fireballs are great at hitting mobs that are swarming you though
Negativity This is a giant slime on the very top of the central mountain of the dungeon. To get there, you need to go through the cave in the back of the mountain, then climb to one of the floating islands. Not all islands are easy paths to the top, so find one you can use. This slime is quite fast, durable, and has a heck of a knockback - so stay away from the edge. A punch bow is good to make him keep distance when needed, and bleed damage from mcmmo swords is a nice touch to get him down.
Labyrinth Dungeon - A maze filled with undead/poisonous mobs, staying alive isn't so easy, as they add the hunger effect. Unlocked with the Knight rank.
Decaying Corpse these are based on the Husk, the desert zombie. They like to swarm you and can kill you faster than you might expect, so be careful. They're quite hardy too.
Decaying Critter These look like cave spiders, but don't let that fool you. Most hate these things, as they're quite fast, move erratically at times, and will drain your health quite quickly if you let them get too close. They're not the easiest to hit either.
Corpse Eater like silverfish, and not as bad to deal with, as typically you won't face more than 2 at a time. The hard part is finding them when you need to kill them for the quest.
The Vault Guardian based off the Vindicator, the axe wielding illager, this guy is only dangerous up close, and vulnerable to arrows. If you do intend on getting that sweet bleed damage, just be careful because he hits hard and has one heck of a knockback. He'll try to corner you and beat you to bits. Keep your distance if you're not up to the challenge. He's found in the very bottom of the maze, if you can find him. He's in a room with 4 pillars with anvils stacked in the back to look like a door, but might not be there when you first arrive. Just outside the room is a spot where Decaying Critters will spawn endlessly, 1 at a time.
Hell Dungeon - Our own little nether fortress, full of nether mobs, and a disagreeable iron golem. Fire resistance might help. Unlocked with the Chieftain rank.
Unholy Ashes are wither skeletons on steroids, but not as hard as the bosses of the same base. Typically come in sets of 3. While there's no quest to kill them at this time, they are a source of wither skulls.
Burning Spirit are the blazes, and aren't much tougher than the base mob.
Living Flesh are the magma cubes, and are surprisingly durable. Only really annoying when they get on top of you. Good source of magma cream.
The Titan is the iron golem boss of the dungeon, found on the main floor, at the back. He behaves like an iron golem - he's neutral to you at first, and attacks any mobs that get too close. However, if you want him for the quest or for the drops, you'll have to aggro him. This guy may be slow, but he has one mean punch - you'll get knocked back and high quite a bit, so be ready for fall damage. You can cheese him with arrows, just don't do it from above or staff may take exception.
Congrats, if you've read this far, here's the hidden gem of the guide.
At the entrance of each dungeon (except the Gates), you'll find an npc named Explorer Laen.
This guy will give you quests that reward you, typically with amberstone.
What isn't commonly known is, his quests are repeatable.
Most of the mob quests can be done every 24 hours. The boss quests are once a week.
At time of writing this, I'm hoping the rewards for these will get looked at again, because it seems some don't pay as well as they should.