Shilo mist can be found killing the zombie variant of undead ones on Karamja. All players within a square of the mist will receive 2-3 damage per hit. To avoid this, players need to be at least two squares away from where the mist spawns.
As the mist does not inflict recurring poison, anti-poison potions are not required.
Historical
Shilo mist was also part of a former random event that occurred while Thieving from chests. Occasionally the chest would become stuck and mist would come out and damage nearby players. Despite the fact it had a green hitsplat, however, the mist did not inflict recurring poison.
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A poison gas cloud. -
The animation which appears several times when spawned.