The Crystal Egg Zone boss is an optional boss battle in the Master System and Game Gear game Sonic the Hedgehog 2. The mechanical arena is the seventh and final Master Robot, operated by Dr. Robotnik in a last effort to eradicate Sonic.
It takes place inside a lightning chamber, which Dr. Robotnik makes extra-fatal by throwing gyrating, flower-shaped energy bolts into the mix. However, the boss actually isn't that difficult if Sonic stays inside the transport chutes until the chamber clears; at which point he can leap out, attack Robotnik and dive back in to safety before the attacks renew themselves.
The good ending in the 8-bit version of Sonic the Hedgehog 2 shows Sonic the Hedgehog defeating Doctor Robotnik. Sonic is upset that Robotnik managed to escape, but Tails teleports in his place and is freed. Tails then smiles at the player, and Sonic shrugs at the camera (his idle animation). The cutscene after that shows Sonic and Tails running through a valley from day to night, while the credits scroll. At the end of the cutscene, an image of Sonic and Tails appears in the stars, showing the bond of the duo.
- This is the first instance of a "True Final Boss" in the series.