Patrol

Patrols, known as pillager patrols $$, are common naturally spawning groups of pillagers and/or occasionally on Hard difficulty, vindicators.

Spawning
Patrols spawn as a group of 5 pillagers $$ or 2–5 illagers $$ (all pillagers except in Hard difficulty, which has a 20% chance for any pillager to spawn as a vindicator instead), one of which is the raid captain. The raid captain wears an ominous banner, known as the  illager banner in Bedrock and Education Editions, on its head. The other illagers follow the leader.

Java Edition
Patrols spawn in any biome (except mushroom fields and its variants) at light level 8 or lower, only at night, like monsters.

Patrols spawn naturally after the world age reaches 100 minutes (5 in-game days), then after a delay between 10–11 minutes an attempt is made to spawn a patrol with 20% chance of success. After an attempt is made, the delay is reset.

Bedrock Edition
$$ patrols spawn around 24–48 blocks away from the player (or more than 48 blocks away from the player if they are in a village) in any biomes except in, , and  biomes. Additionally, patrols can spawn in biomes where common monsters cannot spawn such as in:

Patrols spawn on solid blocks at the surface, like monsters. On Easy difficulty, patrols can spawn only at light level 0–7 on the surface. On Normal and Hard difficulty, patrols can spawn in any light level.

Patrols spawn naturally after the world age reaches 100 minutes (five in-game days), then after a delay between 10–11 minutes an attempt is made to spawn a patrol (20% chance of success), repeating the delay and spawn attempts.

Behavior
Patrols seek villages, wander around, and attack nearby players, villagers (adult villagers only $$), iron golems, snow golems and wandering traders. The patrol members' heads turn to follow a player who looks at them or attracts their attention. A player who approaches within 10 blocks of the patrol provokes them into loading their crossbows and attacking, and pursuing if the player flees. The pursuit lasts until the player dies or gets 10 blocks away.

$$, any illagers or witches that did not spawn with the patrol can join the patrol if sufficiently near a patrol captain. Evokers, illusioners, and witches who join a patrol wander around but do not seek out targets as other illager members in the patrol. Evokers and illusioners do cast spells to attack mobs, however.

If a "Johnny" vindicator joins a patrol, all patrol members assume the Johnny vindicator's aggressive behavior, attacking any non-illager mob.

$$, when one of the patrol members is attacked, it alerts nearby patrol members within 10 blocks to attack the same target as a swarm. The attacker can be any mob and doesn't need to be a player, and the distance between attacker and patrol can be up to 64 blocks away from alerted patrol members.

The patrol captain always drops an ominous banner, known as the illager banner in Bedrock Edition, upon death.

Bad Omen
Upon killing a patrol captain, the player receives the effect for 100 minutes (displayed as infinite $$), which has no consequences unless the player enters a village. Upon entering a village, the effect is removed from the player and a raid spawns nearby after about five seconds. $$, the level of the Bad Omen determines the chance of enchantment on raid member's weapons, and additional waves. The number of waves depends on the difficulty. The effect can be removed by drinking milk.