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. Due to a bug, a percentage of their charge damage is included in the secondary cloud of poison as physical damage; the AoE cloud. This means that one could avoid the initial poison from the spear but still get damage of the cloud should his 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.
Physical damage can be mitigated by equipping items with integer physical damage reduction. Other 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, working on all guest monsters is to Save and Exit the game and try again, hopefully being able to avoid having them spawn again.