Trial Omen is a harmful (negative) status effect in Minecraft Java Edition 1.21.
minecraft:trial_omen
Give Trial 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:
Trial Omen supercharges trial chambers: trial spawners convert to their ominous versions while the effect lasts, unlocking ominous vaults and the best loot.
Enter a trial chamber while carrying Bad Omen and it converts into Trial Omen, or drink an ominous bottle found in vaults and from raid captains for a direct dose. The effect lasts while you stay in the chamber area.
Trial Omen is the hard mode you actually want: ominous spawners are far more dangerous, but ominous trial keys open ominous vaults, the only source of the heavy core for the mace. Bring your best gear and milk to control when the event starts.
The effect ID for Trial Omen is minecraft:trial_omen. Java Edition 1.13+ /effect commands use this namespaced form.
Run /effect give @p minecraft:trial_omen 60 1 to get Trial 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:trial_omen.
Trial Omen is a harmful (negative) status effect in Minecraft Java Edition, based on its registry category (harmful).
Walk into a trial chamber with Bad Omen active, which converts it, or drink an ominous bottle from vaults or raid captains. While active, trial spawners become ominous trial spawners.
It converts nearby trial spawners into harder ominous versions. Defeating them yields ominous trial keys, which open ominous vaults containing premium loot including the heavy core.