Spies!

The spy spawns near large villages, where it appears among a few villagers, and despawns at once once the spawn conditions are met for the boss battle to start. The battle has two phases. In the first, the spy is fully invisible and attacks the player in most situations, while it periodically wanders randomly and triggers abilities that can disrupt the village. When the spy is hit it teleports to another random spot, so the player must find it again, and the only way to track it is by hearing a living sound that matches a normal pillager.

The spy has many abilities, cast in two ways. Primary casts follow a timer and produce a consistent, distinct sound queue, similar to a goat horn. Secondary uses perform a single ability and nothing else, and are controlled by RNG. Each primary cast raises an internal counter by 1, and when the counter reaches 30 the spy calls a raid at its location. A raid can also be started by other means, and the spy enters its second phase regardless, keeping its current health and behaving the same way it does in phase 1, with more health.

Spies! requires Geckolib. It is published for Forge only, for 1.20.1, and runs on client and server, so the server and every player need it. The mod is released under the MIT License and is not open source.