Tomb Vipers are a variation on the Claw Viper monster, usually encountered in Nihlathak's Temple.

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.

To explain:

A percentage of their charge damage is included in the secondary cloud of poison. The AoE cloud. This means that you could avoid the initial poison from the spear but still get the physical + poison damage of the cloud should you run into it.
The cloud is not visible. This is why many people think they get killed by something that isn't really there. The cloud will stay about five seconds after the initial spear's cloud vanishes. If you do not move in this time, the second cloud will not effect you. Getting knocked back will also not trigger the second cloud. You will get poisoned by the viper and get a nice percentage of life taken away from you but it won't be over time.
Best thing to do is stand still when they fire or run past them at high velocity. The cloud will only form a second or two after the spear is thrown, this gives more than enough time for a character to run past them. Should you need to stop... stop and don't run or walk until five seconds after all the clouds have gone. (Source)

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.

Claw Viper
Tomb ViperClaw ViperSalamanderPit ViperSerpent Magus