On Nightmare and Hell difficulty, the Halls of Vaught may spawn with Tomb Vipers. Their primary attack is a poison missile similar to a Plague Javelin, only capable of much more damage. A percentage of their charge damage is included in the secondary cloud of poison; the AoE cloud. This means that one could avoid the initial poison from the spear but still get the physical + poison damage of the cloud should their character run into it. The cloud isn't visible however, but will stay about five seconds after the initial spear's cloud vanishes. If the player does not move in this time, the second cloud will not affect them. Getting knocked back will also not trigger the second cloud.
One way of countering their attack (for melee characters) is by moving close enough to the creatures so that they will melee instead of using their missile attack. Casting Dim Vision on them will work as well as they won't attack something until they're aware of it. Another tactic is to Save and Exit the game and try again, hopefully being able to avoid having them spawn again.