The Level 1 bossfight, disguised as sleeping stone before it wakes.
Before it ever appears as an entity, Skinwalker "sleeps" underground disguised as a deepslate block, spawned at a random location near the player and active only during Corruption Level 1. Left alone, it occasionally relocates itself by breaking and re-placing its own block.
Mining it exposes and activates it: it becomes a full entity and gives chase, following the terrain and refusing to stop at bedrock, it passes straight through. Getting within 3 blocks costs 10 damage.
Throwing a defeated Skinwalker into the void beneath bedrock kills it, the only way to advance World Corruption from Level 1 to Level 2.