From the Super Mario Wiki
Dark Trashures are dark purple treasure chest-like creatures that appear as enemies in Mario & Luigi: Bowser's Inside Story and the 3DS remake Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. They are stronger variants of Trashures that appear in the hijacked Peach's Castle, and are mostly seen in battle with Dark Mechawfuls or Naplocks.
Dark Trashures attack by throwing the contents of their bodies at Bowser and the Mario Bros., much like normal Trashures. However, the items are thrown faster, and there is a higher proportion of harmful items to useful items such as mushrooms and coins; in Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey, they also no longer throw Scutlets at Bowser during their attacks. Like normal Trashures, if the player does not counter the hammer from the Dark Trashure, Bowser may be stunned for a few turns.
They have the highest health out of all non-boss enemies excluding those encountered in The Gauntlet, and are also more prone to running away and denying the player any EXP for defeating them. One Dark Trashure on the second floor of Peach's Castle has a Blitty that Bowser can obtain use the Vacuum Block.
In the original game, while Mario and Luigi cannot normally fight Dark Trashures, it is possible for them to battle one with cheats enabled. Most of its attacks function normally, with the objects being thrown at either of the Mario Bros., and they can be dodged by jumping; however, the cannonball is thrown at such a rapid speed that it is very difficult to predict and dodge beforehand.
Mario & Luigi: Bowser's Inside Story / Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey
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