https://www.youtube.com/watch?v=96Qd_D41YsA

Appearance:

The Ancient Guardian is an oversized elder guardian with a predominantly dark purple body as well as three pink stripes. All of its spikes are a light purple to white gradient, and its eye is pink in color with a white iris. On its back are a few patches of algae, coral and seaweed, with some on its tail. A shard of the Orb of Dominance is lodged in front of the algae. Its face is full of cracks and a long scratch is seen across its eye.

Behavior:

When the hero reaches the arena, the ancient guardian enters by bursting through a wall. After taking notable damage, it can swim away from the arena to avoid further damage. In addition, it has five attacks:

Spike attack:

The ancient guardian can attack nearby heroes or summons by piercing them with its spikes, which creates a high-damage shockwave.

Spike Protection:

The ancient guardian will sometimes pop out its spikes as a means of defence. Attacking it with a melee weapon will result in the attacker taking some damage back in a similar manner to the Thorns enchantment.

More info: https://minecraft.fandom.com/wiki/Minecraft_Dungeons:Ancient_Guardian

Photo by Darick