Megabug

The Megabug is the main antagonist of the Mario + Rabbids series. 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.

Mario + Rabbids Kingdom Battle
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 the Megabug's 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 manages to save Spawny from Bowser Jr. in the Lava Pits, but the little Rabbid soon wanders off, seemingly drawn by a mysterious force. Soon, the party discovers that the Megabug had become strong enough to manifest a physical form, appearing as a flat orange dragon-like creature. It promptly absorbs Spawny and flies 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 him, forcing the Megabug to separate from Bowser. Undeterred, it attempts to attack its foes directly, but Mario's team fires a series of blasts in unison, seemingly destroying the Megabug for good and returning the Mushroom Kingdom to normal.

Mario + Rabbids Sparks of Hope
In Mario + Rabbids Sparks of Hope, it is revealed that Cursa is the reincarnation of a piece that had remained from the Megabug. After it was destroyed, a tiny fragment of it remained, floating aimlessly throughout the galaxy, until it mutated with stellar debris, gaining sentience and desiring to grow with power. Cursa was drawn to the Comet Observatory when it discovered the Rabbids meeting the Lumas. Rosalina did not sense its arrival time until it was too late. Even so, she protected the Rabbids and Lumas, at the cost of being captured and transformed into Cursa while the saved Rabbids and Luma were flung far away and merged into Sparks. Weapon skins based on the Megabug are also available as a pre-order bonus in the Gold Edition.

Names in other languages
Megabug