Awesome but Temporary

Whoa! Look at that awesome new sword -- it has twice the attack of your second best sword, so it must be the Infinity+1 Sword, right? Of course it -- Hey! It just broke in the next cutscene! Apparently it was Awesome but Temporary.

You see, there are three requirements for a weapon (or armor or shield or spell or...) to be Awesome But Temporary:

• It must be clearly better than anything else you have by a significant amount.
• No matter what, the plot will require that it be broken, lost, stolen, sacrificed, etc... or, at least, the dev team planned it that way.
• If you DO get it back, either it will be much weaker than when you first had it, or late enough in the game that it's only slightly better than (or even weaker than) your other equipment.

By definition, there will be spoilers in this trope.

Compare Guest Star Party Member. If you start the game with it, but lose it in the first act, that's A Taste of Power.

Examples of Awesome but Temporary include:

• In the first of Final Fantasy Legend's four worlds, you pick up a powerful sword that has ~50% chance of outright slaying any enemy with a weakness, a piece of armor that resists every element and has 2.5 times the power of the world's second best armor, and a shield that blocks every single attack when used. Naturally, you have to part with them to advance, and you don't find comparable equipment until at least the third world.
• In Oblivion's starting quest, you can loot not one but two Akaviri Katanas from fallen allies, and they're about 50% stronger than the second best sword you can get that early. Unfortunately, they get removed near the end of the quest.
  • Later in Oblivion, you can get the Boots of Spring-Heeled Jack, which raises your acrobatics by a whopping 50 points! Too bad that the developers intended you lose it in the next quest (although there are a few workarounds).
• You get a mighty bazooka for the first battle of Secret of Evermore which does around 200 or so damage per hit. Afterwards, you lose it and you're stuck using a bone and doing a mere 20 damage. Then about 3/4 of the way through the game, you get it (or another identical bazooka) back...and only one shell with no way to buy more. You finally get the ability to buy shells at the very last "town" area before the Final Boss.
• In Sword of Vermilion, you find the stolen spellbook Sanguios, which heals you to full HP and costs only 2 MP. For reference, the only other healing spell available at that point, Sangua, heals 60 HP and costs 6 MP. Then you learn that said spellbook was stolen from a king...
• Alucard starts with his personal equipment in Symphony of the Night, which is among the best in the game. Not five minutes later it gets scattered around the castle, and won't be recovered for a long, long time.
• StarCraft 2's single player campaign is packed with these. Giant drilling laser you can turn against your enemies? Nigh-unstoppable super robot? Gone after that mission is over.
• In Warcraft 3 campaign happens many times:
  • Arthas gains Frostmourne, a sword that deals devastating Chaos damage for the last mission of the human campaign. He never has this ability again, in the later campaigns or the Expansion Pack, even though it is an important plot point that he is still using the exact same sword.
  • During one mission in the orc campaign, your regular Orcs become the much more powerful Chaos Orcs (Grom Hellscream included) who also deal Chaos damage (neccesary to beat a boss immune to every other kind of damage). Not only you never get to control Chaos Orcs again, but you have to fight against them later
  • During the only mission you get Illidan, he'll get a PERMANENT Demon Form which is DEVASTATING (regular demon form only last 60 seconds). Like the Orc campaign, chaos damage is needed to defeat a boss as well.
  • Apart from the campaign, several summons (and summoning items) exist, with a duration of anywhere from thirty seconds to three minutes.
• Yet another Blizzard example are the Weapons in World Of Warcraft Raid Tempest Keep : The Eye, Which has 4 weapons and shields which are way better than anything available at that level, but are not able to be used outside of that raid as they will disintegrate. e.g. weapon: http://cata.wowhead.com/item=30313#dropped-by
• City of Heroes hands out quite a few Temporary Powers in its various arcs, all either limited in duration or in number of uses. How awesome they are varies widely according to the character that gets them, but some, like the powers that do "Special" type damage to particular enemies, are VERY useful as long as you're in the correct level range. A shining example is the Divining Rod, which does a truly absurd amount of damage to Ghosts, yet comes with only 20 charges and gets taken away after a single mission.
• Dark Cloud - You get a sword that doesn't disappear when broken until you fight a mini-boss, so you can theoretically grind the sword up to absurd levels before fighting said boss.
• Tales of Vesperia gives us the mighty Dein Nomos, which allows Yuri to learn the Guardian Field Arte, as well as his Mystic Arte, and grants an excellent bonus to all stats except HP and TP. Duke takes it back not long after you borrow it.
• In the first major dungeon in Lands of Lore: The Throne of Chaos, it is possible to acquire a weapon that adds somewhere around +40 to your attack stat (for comparison's sake, most of the other weapons you have access to at that point in the game are in the range of +7 to +15 or so). At the end of the dungeon, you must give it to the reclusive oracle who makes his home there if you want to advance the plot. Much later in the game, you can find the same weapon available for sale in a blacksmith's shop for the ridiculous sum of 3,000 Crowns (about ten times more than what any other weapon in that area costs, and more than you're likely to have on hand at the time). By then, however, you will have picked up considerably stronger weapons along the way--plus, if you do buy it from him, he won't buy it back!
• RuneScape: At the end of "While Guthix Sleeps", you get to use the Stone of Jas to boost all your stats to insane levels during a fight scene, but Lucien steals the Stone and the boost wears off as soon as the fight ends.
• In Cobra Mission, you start out with a 44 Magnum with seemingly unlimited ammo. Unfortunately, it turns out that the ammo is only unlimited until you finish the numerous mooks attacking you in the first battle. After that fight, you run out of bullets and don't find any more that fit that gun until more than halfway through the game (and even then in limited supply), forcing you to find and use other weapons for most battles.
• In Rayman 2, Rayman at one point gains the ability to fly (rather than just glide down) with his Helicopter Hair, but he loses it again just before that level's Boss Fight.
• Majora's Mask has the Giant's Mask, which you can only use during the battle with Twinmold.
• In Lufia and The Fortress of Doom, you obtain the Dual Blade remarkably early in the game, and it outclasses every other weapon up to that point ... but it ends up being broken in a later encounter.
• In Star Ocean, you are tasked to rid a castle armory of a monster infestation. You can borrow the powerful weapons and armor in there, but have to give them back (one of your characters mentions her disappointment at not being able to keep them) once you leave. Item Crafting features heavily in the game, though, and the two swords you get there can be crafted into something even more powerful. If you do upgrade them while you have them, you don't have to return them; otherwise, they are Lost Forever.