Learn English – word/phrase to describe an action which leads to it being pointless

I propose fateful, as in Aladdin's fateful decision or Juliet's act on that fateful night.

Although it can mean simply "significant" or "determined by fate," fateful carries connotations of calamity or misfortune.

http://www.merriam-webster.com/dictionary/fateful

1: having a quality of ominous prophecy [a fateful remark] 2 a : involving momentous consequences : decisive [made his fateful decision to declare war — W. L. Shirer] b : deadly, catastrophic

More poetically, to express a stronger sense of menace or wickedness, one might use baneful or baleful.

The term you are looking for in gamer parlance is "Key Item".

Key items are special items that players can only obtain once, and either aid the progression of the storyline or allow access to new areas. - Bulbapedia

The reference is specific to the Pokemon franchise, but the term is valid in any video game. Another accepted term is "Quest Item", and this would be particularly true for your case since the item is part of a series of quests.

Quest items are objects needed to complete a specific quest. Usually they have to be given up when the quest has been completed - Skyrim Wiki

There are many examples of these types of items in video games, but the general thread between them is that they all progress the story or allow access to another location in the game.

Note that these are both specific to gaming and would probably be considered gaming-specific jargon. While they might be understood by those who understand the context, they are not likely to be recognized as commonly accepted expressions in the english language.

The specific premise that you have offered as an example suggests that the item in question is not necessary to advance in the game. The implication being that a Warp Drive is not actually necessary to travel to another planet.

If it is not, you would simply modify it as a False Key Item or False Quest Item, though this depends on whether the game is intentionally trying to decieve you, or if it is simply a normal item that is given as a reward for your quest (which you might call a "reward item").