The Candle is an item that can be found in Parapa Palace. It can be used to light up caves, making it much easier to navigate them. Without the Candle, the caves are pitch black and enemies inside are virtually impossible to see. The Candle is not needed to beat the game, but it is very useful.
The Candle can be obtained by entering Parapa Palace and defeating a Guma and orange Iron Knuckle. It is located in the basement of the palace, and must be claimed before proceeding to the other palaces. Once acquired, the item will automatically illuminate caves, allowing Link to see any enemies within them. This makes it significantly easier to navigate the many caves that fill The Adventure of Zelda II, especially the longer ones.
It also has an unintended side effect: certain projectiles, such as Octorok rocks and Goriya boomerangs, will flash Sprite Palette 0 every six frames while they are being fired. This is an ode to the fact that these items were designed for use in a non-graphical environment, as the original NES title featured a warning on the title screen that said “PUSH START”.
Top any% runners have experimented with several different ways of using the Candle to speed up their runs. However, the most common strategy is to get it as early as possible and then skip using it in subsequent palaces. This allows a run to start with an advantage of nearly ten seconds over a full-speed run that does not use the Candle.