Spoiler warning: Plot and/or ending details follow.
Thermite was a highly explosive substance.
Followers of Kodos stored a case of thermite grenades at his encampment on Tarsus IV that they fled to after they murdered 4,000 colonists. These grenades had either been smuggled in by a follower of Kodos or had been left over from before the ban.
During a fire fight between Starfleet Lieutenant Commander Gabriel Lorca and Ian Galloway, a stray phaser shot from Lorca struck the case of thermite grenades. The resulting explosion killed Galloway and fellow Kodos supporter Fiona Okafor, burning their bodies beyond recognition. As Kodos had earlier switched his DNA records with those of Galloway's, genetic testing showed the body as belonging to Kodos. (DSC novel: Drastic Measures)