Cave Boss- Crystal Golem

Currently Minecraft seriously lacks in Bosses and good reasons to fight those bosses. The two current bosses, the Elder Dragon and the Wither are ultimately pointless as they do not reward you in anything worthwhile (beacon is better for decoration than anything else). This suggestion includes a new challenge to face in the game while also providing a drop that actually unlocks things you would want, and it builds off the Cave Update going on.

Giant amethyst caves would exist in the deepslate layers far below ground. In them you find a central, massive amethyst cluster that is a different color (lets say red). This specific cluster can only be mined with a diamond pickaxe and when it is it comes alive as a massive golem.

This behemoth would have a variety of attacks at its disposal, creating damaging stalactites at the player's feet, shooting crystals (making a shield very useful), a ground pound with knock back, etc.

Upon death it drops living crystal shards. Initial idea is you can create a Great Crystal Hammer with these that have a big AOE knockback attack. Or create a Crystal Shield that is more powerful and also causes knockback on blocked melee attacks. 

For a late game item, you could create a drill made out of living crystal shards and netherite that mines a 1x9 area all at once. At that point you're far into the game and it would be a fantastic reward.