The best Fallout: New Vegas mods (opens in new tab) fill its casinos with NPCs, its thoroughfares with traffic, and its streets with new weapons. With the latter in mind, let me vouch for Cloud Grenades—a new project from Hopper31 that inserts the toxic gas clouds from New Vegas' Dead Money DLC (opens in new tab) into grenades. The results, as the header image above suggests, are predictably gruesome.
Designed for those moments "when you want to kill someone as painfully as possible" (we've all been there, right?), Cloud Grenades turn the blood red noxious mist into a throwable bomb.
"There are four separate tiers, ranging from just throwing a bottle of cloud residue which deals 25 damage, up to Lethal Cloud Kiss which deals 169 damage," explains its creator (opens in new tab), before noting Ghost People and those wearing the hazmat suit from Old World Blues are immune to the Grenades' attack, true to the base game's lore.
Everyone else, however, is less likely to survive. "Everything is fair game," adds Hopper31, "and anything killed [can] be reduced to a pile of cloud residue". Naturally, said ashes can be harvested and turned back into more grenades. War never changes, etc.
Hopper31 continues: "These can be obtained either from converting an existing cloud residue/cloud kiss poison into the weapon version at a workbench, or cooking the throwable version directly at a campfire. They can then be converted back to their base game counterparts at a workbench if needed for anything else."
More information on Hopper31's Cloud Grenades, including installation instructions, can be found on its Nexus Mods page (opens in new tab). Here's a handful of screens: