Z-Saber (Mega Man X3)

Mega Man X can use the Beam Saber in Mega Man X3. To obtain it, Zero must pass through the one boss door that he can enter in the entire game. It's found on the second station of Dr. Doppler's fortress, and is the first door in the level. It leads to the sub-boss Mosquitus. Defeating the sub-boss with Zero will cause him to be severely injured by its explosion. Zero will then pass the Beam Saber to X before teleporting out to Dr. Cain's lab, becoming unavailable afterwards. Of course, losing Zero results in the "bad" ending. To use the Beam Saber, X must fully charge his buster - it does optimal damage and is considered the game's ultimate weapon.