The Level 2 bossfight, tied to the Moon events.
The Broken One emerges from the meteor that opens the Broken Moon event, the Corruption Level 2, 16-night cycle, or from the Corruption Level 2 random spawn pool.
Eye contact reduces the player's render and simulation distance to just 2 chunks, and it begins actively targeting and chasing once within 96 blocks.
Getting caught at close range is an instant kill. In unsafe mode, being caught also permanently bans the player from the world, it writes a file to their computer and changes their desktop wallpaper, before kicking them.
Throwing a defeated Broken One into the void kills it and advances World Corruption from Level 2 to Level 3, the final countdown.