The Skeleton King

Summoning
It rises once per Halloween in an urban or suburban area.

During Battle
This boss lumbers around wreaking havoc with its warriors. Any Skeleton Warriors nearby will flock to it. Spawn rates of Skeleton Warriors are higher around this boss, and they can spawn in abandoned buildings, on normal caves, graveyards, etc. After defeating it, there is a 3% chance that it will absorb the energy of nearby monsters and humans to enter a second phase where it is much more powerful.

Tips
It is best to not fight this boss alone, especially in phase 2.

Trivia

 * This boss is widely considered the final boss of Halloween.