Thalaron radiation is a form of electromagnetic radiation in the tertiary EM band that was deadly to organic forms of life as it was capable of instantly killing such organisms with even small amounts.
Unlike gamma rays, thalaron radiation did not bring about a mutative effect to a living organism's genome but rather destroys it through a form of necrosis. Once every cell has been exposed to the radiation, the once-living being simply turns to ash and fractures.
The Vulcan people during the time of Surak, most notably Varekat of the Vulcan Space Institute, had attempted to experiment with this type of radiation in order to use it as a weapon to be used against rival factions, however, nothing came of the research at the time. This research would be recalled when the Exile fleet from Vulcan reached what would be the Romulan world of Remus. The planet possessed numerous ores that emitted different types of radiation and it was calculated that one of those could be thalaron radiation. (TOS - Vulcan's Soul novel: Exiles)
The knowledge of such a type of radiation was only theoretical to Federation scientists, with the radiation being considered a form of biogenic weapon. The Romulan Star Empire government under Praetor Narviat experimented on banned thalaron radiation research with a Tal Shiar facility being constructed on a remote world for study of the implementation of a thalaron generator, however, new factions with the Imperial Senate stopped the project from reaching its final stage. (ST - Tales of the Dominion War short story: "Twilight's Wrath"; TNG movie: Star Trek Nemesis)
Shinzon, a rising warrior within the Imperial Romulan Fleet and part of a failed experiment in replacing Captain Jean-Luc Picard of Starfleet, managed to defeat a Dominion occupation force at the Tal Shiar compound and save the thalaron generator for his own purposes. Working with Senator Tal'Aura, a small amount of thalaron particles were released into the Imperial Senate which resulted in their death and allowing Shinzon to achieve the title of Praetor. He also installed the thalaron generator on his warship, the Scimitar which he planned to use against his ancestral homeworld of Earth but this plot failed. (ST - Tales of the Dominion War short story: "Twilight's Wrath"; TNG movie: Star Trek Nemesis)
Following Shinzon's failed attempt at genocide (and Data's subsequent sacrifice), there was growing fear within the quadrants of a thalaron war which would devastate all civilizations. A study published by Memory Alpha suggested that if the Romulan Star Empire collapsed then it was possible that within three years the quadrants would be devastated by thalaron weapons which would weaken all the factions against a possible renewed threat from the Borg Collective. (TOS novel: Captain's Blood)
Thalaron weapons were eventually banned by all major powers in the Alpha and Beta Quadrants. During the Borg crisis of early 2381, however, Seven of Nine proposed building a thalaron generator for use against the enemy armada. This plan was initially rejected by Starfleet Command, but after seven thousand Borg cubes broke through Starfleet's line of defense in the Azure Nebula, Captain Picard briefly entertained the idea. He was persuaded to scrap the plan, however, after Geordi La Forge refused to assist in building a thalaron generator. (ST - Destiny novels: Mere Mortals, Lost Souls)
In 2409, the Romulan colony of Delta Corvi contained a planetary Thalaron bomb located beneath the settlement which was to be used as a desperate last weapon against invaders. At the time, the world was attacked by the Tal Shiar with assistance from the alien Elachi. A spy from the Romulan Republic who had infiltrated the Tal Shiar managed to warn Major Vrimak who headed the colony. In desperation, he intended to detonate the Thalaron bomb to prevent the colony from falling into the hands of the enemy. However, he allowed for the colonists to be first evacuated whereupon the weapon was activated leaving Delta Corvi a dead scorched planet. (STO mission: "Cloak and Dagger")