Have you ever wondered why there are so many things that you can hold as long as they don't exceed the Cap, but for those things from event items and items Too Awesome to Use, to all things that's important, there's only one in the entire setting? It's simple — there can be only one of them.
When you want to show that something is really that rare or precious, they'll not be just limited in number, but there's only one of them present in a universe setting. MacGuffin and quest items, in particular, always has one only, and nobody would even consider finding a replacement when it's lost, destroyed or stolen, at any circumstances. They will, and must get that one back, in one piece.
While this trope is omnipresent, the most time you would recognise this would be in video games, especially RPG games. Since you can't have everyone sharing one precious item or Public Domain Artifact, you'll have to consider how they should be distributed among your party. For some game, even if an item's a monster Random Drop, the game will tell you that it'll not drop for a second time, and the shops will tell you that they only have one of them in stock, and it'll never replenish at all.
It's worth noted that some modern games averted this trope already, by generating random gear instead of giving unique items. Also, if you use cheats to duplicate items, in most times you can have more than one of them, hence averted and invoked it. And it can be justified if they're obtained from methods which can't possibly give you more of them.
Compare One-Steve Limit, Uniqueness Value, Unique Enemy, Power Equals Rarity and It Only Works Once. See also Infinity -1 Sword. Public Domain Artifact are almost certain to be unique items. Lots of Permanently Missable Content is caused by this.
Note: This trope is not about items that are informed to be "unique", even inside a work. This trope is about making items and objects one-of-a-kind in-universe to show its importance, even if that's unreasonable.
Pages with their own examples:
- In early editions of Dungeons & Dragons, most magic items were generic and you could find any number of them.
- Artifacts and relics were unique: only one of each of them existed in a game universe. Thus there could be only one Eye of Vecna, Codex of the Infinite Planes or Ring of Gaxx.
- Some magic items of less power than artifacts and relics were also one of a kind. For example, in the Forgotten Realms there was only one Albruin (sword), Reptar's Wall (shield) and Mierest's Starlit Sphere.
- Atelier Iris has a few ultimate items. Each of them can only be made once, even through different playthroughs of the game.
- In Castlevania, the most powerful weapons are often in secret rooms or obtainable after boss fights. In Aria of Sorrow, the souls of the bosses and a number of other creatures are only obtainable once.
- Lots of unique items in the Dragon Age series: usually when something has a proper name, you won't find a second copy of it (except by exploiting bugs or the save import). DA actually combines randomly generated and pre-determined loot for ordinary and unique items, respectively.
- In Dragon Quest VIII, a few items can only be made once or twice. The Liquid Metal Sword is one, the Sage Stone, and Timbrel of Tension is another.
- The Elder Scrolls series has a great number of these in each game. Daedric artifacts are the most evident, but there are many other examples of quest reward items and unique NPC equipment which fit, many of which have their own textures. The Unofficial Elder Scrolls Pages maintains a list of unique items for each game.
- Fallout 3 and Fallout: New Vegas both have at least one unique variant of each weapon which has better stats or a unique ability.
- Half-Life 2 has the Stun Stick, which is used as an energy pickup for your HEV suit. The only time it appears as a pickup is at the beginning of "Route Kanal", where it is used by a metrocop who drops it when you kill him. It only gives you seven points of energy.
- Any items dropped by bosses in Marvel Ultimate Alliance count as this. There are also slightly more common character-exclusive items, which boost a stat and a certain attack for them. The sequel replaces all this with equipped medals that boost stats for the whole party.
- In the Pokémon handheld videogames, certain Pokemon (the most well known of them being Mew, Mewtwo and the fossils) don't appear in the wild and you only have one shot at obtaining them during a scripted event. Some items like the Master Ball are only given to you once.
- The Voice Box is a usable inventory item which only appears once in the game. It can be found in "Rrajigar Mine", behind a pillar in the area with the first Nali. It serves no real function other than a noisemaker, which the enemies completely ignore anyway.
- In the last level, right before the Final Boss, there is a pair of Super Jump Boots, which last forever. They use the same model as the regular Jump Boots.