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Specifics: In-game screenshot.
The Boom Rocket is an item in Kid Icarus: Uprising. When used, it sits on the ground for a moment counting down, then launches into the air. After a short time, it falls back to the ground and causes damage to anyone unfortunate enough to be in the enormous blast radius. The time delay makes it tricky to use effectively, but can be useful in crowded areas.
The Boom Rocket is found in certain chapters as a usable item, usually in an area with plenty of enemies. It's first appearance is normally in the second round of the ambush in Chapter 3, Heads of the Hewdraw. It can also be found randomly in Together Mode, where it is slightly trickier to use against player controlled opponents.
These rockets first launch skyward, after some time falling back down and unleashing a massive explosion when they land. The delayed attack makes them a bit tricky to use but effective in areas where foes are likely to gather.