Spawner Golem/Dungeon Golem

This would be a mini-boss spawning in larger dungeon structures, called Crypts. Small crypts would generate with a door to the surface, but only 1 room, the miniboss room. Larger crypts would generate underground, so they are harder to find. However, they have more rooms with more loot. All the rooms eventually connect to the miniboss room in the center. Upon entering the miniboss room, a single spawner in the floor would transform into a spawner golem, a brute as tall as an iron golem and has skin made of stone bricks. It would hit players with its fists, dealing major knockback, and if the player is close to a wall, kinetic energy too. Sometimes it will roar, and the monster spawner on its head will spawn a small group of 2-4 zombies and 1-2 skeletons. It is immune to arrows unless they hit it's spawner head, tridents, fire, drowning, and fall damage. It can only be harmed by melee attacks and the occasional well-shot arrow.

However, it only has 80 health, and can't attack very quickly. SO if a player is swift and attacks at the right moment. Killing it shouldn't be a problem. When it dies, it will unlock all naturally generated chests in the radius of the crypt/dungeon. This doesn't affect chests that the player places. 

Killing the spawner golem would be a good incentive to players as it gives them really good loot, sometimes even blaze powder for potions or eyes of ender. As for the other rooms in the crypt, I'll leave that to Mojang to decide.