Raid Omen is a harmful (negative) status effect in Minecraft Java Edition 1.21.
minecraft:raid_omen
Give Raid Omen II for 60 seconds (Java Edition 1.13+; duration in seconds, then amplifier where 0 = level I):
Remove the effect from the nearest player:
Raid Omen is the short countdown form of Bad Omen: when it expires, the raid on the village you just entered begins.
Enter a village while carrying Bad Omen and the effect transforms into Raid Omen for 30 seconds. When the timer ends, the first raid wave spawns, so this is your last window to prepare or leave.
Use the 30 seconds deliberately: ring the village bell, wall off villagers, and position yourself at the spawn side. Milk during Raid Omen still cancels the raid if you change your mind.
The effect ID for Raid Omen is minecraft:raid_omen. Java Edition 1.13+ /effect commands use this namespaced form.
Run /effect give @p minecraft:raid_omen 60 1 to get Raid Omen II for 60 seconds. The first number is the duration in seconds; the second is the amplifier (0 = level I, 1 = level II). Remove it with /effect clear @p minecraft:raid_omen.
Raid Omen is a harmful (negative) status effect in Minecraft Java Edition, based on its registry category (harmful).
It is the 30-second transformation of Bad Omen that occurs when you enter a village. When the timer runs out, a raid begins at that village.
Yes, drinking milk during Raid Omen clears it and cancels the pending raid. Once the first wave spawns, the only way out is fighting or abandoning the village.