Darkness is a harmful (negative) status effect in Minecraft Java Edition 1.21.
minecraft:darkness
Give Darkness II for 60 seconds (Java Edition 1.13+; duration in seconds, then amplifier where 0 = level I):
Remove the effect from the nearest player:
Darkness dims your screen in pulsing waves, slashing visibility in the deep dark where the Warden hunts by sound.
Sculk shriekers and the Warden apply Darkness when you trigger them in the deep dark, and the effect pulses on a cycle while you remain in range. No potion exists; it is the signature hazard of ancient cities.
Darkness is your warning system: when the screen pulses black, a shrieker has noticed you and the Warden may follow. Move on wool, crouch, and retreat out of range rather than pushing forward blind.
The effect ID for Darkness is minecraft:darkness. Java Edition 1.13+ /effect commands use this namespaced form.
Run /effect give @p minecraft:darkness 60 1 to get Darkness 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:darkness.
Darkness is a harmful (negative) status effect in Minecraft Java Edition, based on its registry category (harmful).
Sculk shriekers and the Warden apply it in the deep dark. It dims your vision in pulses while you stay in range, signaling that you have alerted the area defenses.
Milk clears it, but the shriekers will reapply it while you remain in the deep dark near active sculk. The practical answer is distance: leave the area or silence the shriekers with wool or by breaking them.