A bio-weapon derived from Celatid venom
The C-2M Exotic Chemical Warfare Delivery System (ECWDS) grenade isn't anything terribly special to look at. It resembles a typical AN M14 TH3 incendiary grenade, but a little more high tech and jazzed up. It has a more heavily reinforced looking case, with an indented section in the middle. There is a very prominent 'biohazard' warning label, and two yellow stripes than run along the upper and lower halves.
Celatids are extremely dangerous biological terror agents used by the Mutons, both for their ability to passively scan for and sense human brain waves across large distances, and for their deadly venom. It makes the acid blood from Xenomorphs in Aliens look downright tame. At the same time, Celatids themselves look like big floating blobs. It isn't exactly the sort of thing that inspired terror in the human heart.
The C-2M Exotic Chemical Warfare Delivery System weaponizes the venom into a kind of pure universal solvent. Highly concentrated and extremely volatile. The grenade itself can be thrown or used with the launcher; on impact, it releases a cloud of the stuff out to around three meters. The result is rapid degeneration of any matter in the vicinity. Metals and synthetic materials undergo rapid transmutation; complex organic molecules are instantly broken down. An organic target, if hit, will be both liquefied and evaporated, leaving a toxic puddle of semi-solid matter behind.
The weapon requires a counter-agent to sterilize the area before use and make it safe to pass through again. Traces of the agent will linger in the air for about a minute, but it's pretty heavy and rapidly falls to the ground. User’s NBC scrubber will protect them against it for a while but breathing this stuff will burn it out very quickly. This can and will eat through your boots and you won't feel it until the soles of your feet disappear.