The first game is almost completely disconnected from the continuity of the main series, involving the Avatar Dungeon Crawling through the eponymous Abyss with the goal of rescuing a baron's daughter. The sequel is more closely connected to the series, featuring NPCs from previous games and a storyline that takes place between the events of Ultima VII and Ultima VII Part II, and which fills in a lot of the Guardian's backstory.The Underworld games are notable for being early examples of true 3D environments, predating quasi-3DFirst Person Shooters such as Wolfenstein 3D and Doom.
Tropes present in the Ultima Underworld games include:
Aborted Arc: In an obvious Sequel Hook, in the second game Fissif claims that former Fellowship members are gathering on Buccaneer's Den. What they are plotting there is never revealed, as they didn't make an Underworld 3.
Calvinball: The game of White-Rock-Black-Rock becomes this when you play it in the Ethereal Void. Valid moves include taking the fish, squashing peas, and moving to Limbo.
Colony Drop: Killorn Keep, a flying castle, can be made to suffer this fate.
Continuity Nod: In Stygian Abyss, there is a troll who asks you your name. No matter what you tell him, he says your name is too hard to say and he'll just call you "Rodriguez". In the ethereal void in the second game you come across a troll, who immediately says "Rodriguez! What are you doing here?"
Death Is a Slap on the Wrist: In the first game, if you've planted the Silver Sapling then you return to it when you die. In the sequel, death is permanent in Britannia, but if you die in an alternate world (where you spend the majority of your time) you return to the blackrock gem.
Though you can reincarnate indefinitely provided the above mentioned conditions are met, you're slapped with a reduction to experience points per death.
Defictionalisation: Apparently, the Florida Department of Corrections encourages security guards to live by the virtues of the Guardian as outlined in Underworld 2. Evidently, no one told them that the Guardian is the villain, and that his virtues are the groundwork for a fascist society.
The Dev Team Thinks of Everything: In the first game, you can refresh burnt-out torches and lanterns if you find some oil, or combine a pole and some giant spider silk to make a fishing rod. You can also make popcorn by using corn on a torch (no, it has no purpose, but it's fun).
Door To Before: After reaching the blackrock gem, a convenient passage takes you back to the castle via a portcullis that's locked from the other side. A good thing, as otherwise you'd do a lot of wandering up and down through the sewer.
The Dragon: Mors Gotha, the anti-Avatar who serves as the final boss in Underworld 2, is the Dragon to series Big Bad The Guardian.
Fantastic Racism: The first game actually gives an in-depth examination of this concept, as the settlement in the Abyss was intended to see if humans, mountain-folk, goblins, trolls, and lizardmen could live together. It wasn't entirely successful; the settlement collapsed and most of the groups live in their own sections, though still trade occasionally. Interestingly, the green and grey goblins hate each other more than any other group. Also, it turns out that the lizardmen, far from being evil monsters, are actually a quite intelligent and civilised race who have historically been seen as monsters due to their physical inability to speak the Common Tongue. It may be unintentional, but lizardmen are one of only two races who never attack you without provocation (the other is the mountain-folk).
Game-Breaking Bug: If you fail to adequately handle the "strike" situation with Lord British's kitchen staff in the second game, the game will become Unwinnable. There's also no indication at the time that you've chosen the wrong option on the Dialogue Tree, it isn't until a bit later that you realise you can't progress any further... yet you still aren't sure why unless you consult a walkthrough.
Another bug, which along with the above was fixed in a patch, resulted in Relk attacking you when you first spoke to him, and the whole of Killorn Keep turning hostile towards you. (He's supposed to attack you alone in his quarters later on.)
Incredibly Lame Fun: The troll game of "White Rock Black Rock," in which you are challenged to pick the correct rock... from a pair of rocks clearly visible to both players. You can make it even more difficult by giving the troll a gray rock.
It's more of a puzzle than it appears to be though, because the reward comes from figuring out how to let the troll beat you.
Lethal Joke Weapon: Sort of—they're not jokes per se, just sneaky. Some of the most powerful weapons and armor in Underworld 2 look just like the weakest. There's a hatchet of smiting that's probably the most powerful weapon in the game, a cudgel that opens all locked doors and chests, and a leather vest that makes you immune to fire, to name a few of the most notable.
Lord British Postulate: Of course. The game doesn't allow you to kill Lord British, but you can kill anyone outside of Castle Brittania by attacking them, and, more to the point, you can do it by pushing them into water and completely avoid retribution. Additionally, the Armageddon spell allows you to kill everyone.
Made of Explodium: The bats, which inexplicably burst into flames when you kill them.
The Maze: Most areas of both games have a maze-like quality to them, although the map prevents this from being a problem.
Mirror Universe: Killorn Keep, a mirror universe of Britannia in which the people have embraced the Guardian's leadership.
Missing Secret: The big double door on the ground floor of the Prison Tower, which really looks like it should be openable.
Also, you're told that there are eight pieces of the map of Praecor Loth's tomb. Only seven exist; it's implied that the eighth was destroyed.
Modest Royalty: Lord Goldthirst actually gets offended if you are too obsequious to him.
Money for Nothing: A borderline case in the second game. Merzan provides some pretty useful things, but aside from him there's virtually nothing for you to spend your money on that can't be found abundantly on the floor. The first game is fairly similar, in that the only things you really need to buy are things you could just as easily murder the holder for.
Mushroom Samba: The dream plant which sends you to the Ethereal Void when you next go to sleep. Also the mushrooms, which distort your vision.
Near Victory Fanfare: II has three battle themes: one when you're close to victory, the standard one, and one when you're close to defeat.
Nice Job Breaking It, Hero: The sorcerer villain of the first game planned to sacrifice the Baron's daughter — but only as a means to complete the ritual necessary to prevent an immortal, invulnerable demon from entering the world. When you kill the wizard and save the comely maiden, you unintentionally task yourself with discovering a new means to hold back a demonic, world-destroying murder-machine.
More to the point, if you read his notes, you'll learn that he was capable of bringing back the dead, so it wouldn't even have been a meaningful sacrifice anyway...
Which is a case of Fridge Brilliance, as 'dwarf' actually also means 'an unusually short person'. There is nothing unusual in the size of the Mountain-Folk, regardless what some strangely overgrown humanoids might think. For them, the term 'dwarf' is like 'black' to an African or 'squint' to an Asian.
Nostalgia Level: The maze in the Ethereal Void drawn in crude white lines and populated by stickmen was intended to mimic Akalabeth, the precursor to the Ultima series.
100% Completion: The Tym rune seems to serve this purpose in the second game: only one exists, and it's hidden in an unmappable area full of Guide Dang It moments.
Our Liches Are Different: For once, liches aren't necessarily wizards. Underworld 2 features a trio of liches—a warrior, a wizard, and an assassin. The warrior is the leader, and the most dangerous.
Plot Coupon: There are eight artifacts in the first game, corresponding to the Eight Virtues, and which are necessary to defeat the demon.
Some of them are quite useful, such as the Sword of Justice (mentioned above, it's an unbreakable, relatively strong sword in a game where repairing equipment is the primary money sink), the Shield of Valor (another unbreakable item, it's as good as a high-level shield), and the Taper of Sacrifice (a light source that will never burn out, though it provides the least amount of illumination other than darkness). The rest are just nice to have, but don't do anything.
Power Nullifier: The Guardian's spell has this effect on Britannia to an extent: any spell above the fourth circle (there are eight) cannot be cast, although wands, scrolls and potions still work fine.
Schmuck Bait: The first game has a dead end with a switch and a warning not to flip it. Doing so summons a rat for each flip. The rats do run out eventually, but you can summon a lot of them.
Self-Imposed Challenge: Try to pass out from exhaustion (the Avatar, that is... it doesn't count if you fall asleep at your computer, which will probably happen first). Even getting your wakefulness down to "drowsy" takes half the game.
Sequence Breaking: The Plot Coupon in Talorus is intended to be obtained by completing a Chain of Deals, after which you get plonked on an island. However, you're more likely to stumble upon this island serendipitously.
Useless Useful Skill: Half the skills are either useless or thoroughly redundant. To illustrate: you can put points in Repair, enabling you to keep your weapons and armour in tip-top condition. And you can train in lockpicking, so you can get past those pesky locked doors/chests. Awesome! Except if you'd put those points into mana and casting, you'd have got spells to do both of those and a whole load of other things.
One of the cool things about these games is that there are multiple legitimate ways to do everything. Want to open a door? You can bash it, pick it, cast a spell, find the key, or, if you invested in Lore, find a Cudgel of Opening. All of these represent different potential characters taking different paths through the game—though like all Ultima games, Underworld is rather unfriendly to thieves.
Wallet of Holding: Averted. Gold is about as heavy as you'd expect it to be in real life.
Wizard Needs Food Badly: You have to eat from time to time, but it's pretty much impossible to starve to death without actively trying to do so. The main negative effect of hunger is that it reduces or eliminates the rate of health regeneration when the protagonist sleeps.