Raid

A raid is an in-game event in which waves of various mobs, mainly Pillagers, come to the border to attack enemy frontlines.

There are 2 possible ways for it to happen.

1. It is triggered when a player with the Bad Omen status effect enters Buthistania.

2. When a player with Bad Omen effect enters a 3x3x3 subchunk region around a village center, the effect disappears and the raid starts.

If you are looking for number 2., you should visit Illager Raids.