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The Megabug is an antagonistic entity appearing in Mario + Rabbids Kingdom Battle. It appears to be an anomaly in the fabric of space-time created when Spawny accidentally struck the Rabbids' Time Washing Machine with a blast from the SupaMerge, transporting the Rabbids to the Mushroom Kingdom while warping the very essence of their destination to incorporate elements of the Rabbids' world.
Initially, the Megabug appears as a large blue vortex swirling over the land, occasionally emitting square pixels and lines resembling a circuit board. Over the course of the game, it grows larger and larger, to the point where it practically covers the sky - it also changes color and becomes noticeably more volatile. Beep-0 deduces that its energy is suffused into all of the corrupted Rabbids wreaking havoc on the Mushroom Kingdom; whenever a foe is defeated, it splits into its components (usually a Rabbid and something else it was fused with) before sending the residual energy to the Megabug. With Bowser Jr. abusing Spawny's power to make an army of SupaMerged Rabbids, the Megabug has a theoretically limitless source of power to feed on - the team's mission is to retrieve Spawny before the vortex grows too powerful.
Eventually, the team manage to save Spawny from Bowser Jr. in the Lava Pits, but the little Rabbid soon wandered off, seemingly drawn by a mysterious force. Soon, the party discovered that the Megabug had become strong enough to manifest a physical form, appearing as a flat orange dragon-like creature. It promptly absorbed Spawny and flew off to Bowser's Castle, creating waves of minions (including Bwario, Bwaluigi, and the Lava Queen) to stall its enemies' progress.
After the team reaches Bowser's Castle, the Megabug merges with Bowser himself, creating MegaDragonBowser. Mario and his friends are able to defeat it, forcing the Megabug to separate from Bowser. Undeterred, it attempted to attack its foes directly, but Mario's team fired a series of blasts in unison, destroying the Megabug for good and returning the Mushroom Kingdom to normal.