The Fleet Flask is one of Plague Knight's potion Arcana. It is found in Clockwork Tower and is traded for the Mobile Gear or bought for 6800 gold. Once Plague Knight has taken a drink of the Fleet Flask, he charges forward until he collides with a solid object (except for dirt blocks that break upon being hit), turns around, or throws bombs. In this state he can still jump in order to avoid obstacles and pits.
It is also obtainable as a Relic by Custom Knight, costing 5 magic to use. It has the same effects as Plague Knight's version.