Minecraft wiki entry for Development_versions, associated text: Triggered when a player enters a village with the Bad Omen status effect applied to them.
A large group of pillagers that come to attack villages and kill the villagers.
Mobs appear in waves – the amount of mobs and length of waves is increased per Bad Omen level.
A raid boss bar shows the player how many enemies are left.
A sound similar to a battle horn goes off before each raid wave.