Bombing of Telos IV

The Bombing of Telos IV was one of the first "battles" of the Jedi Civil War. Meant by Darth Revan to be conquered, the civilian target was attacked without warning or provocation by former Republic Admiral Saul Karath under Malak's direct order, marking the beginning of the conflict.

The attack
To prove his loyalty to the Sith, Republic defector Saul Karath, captain of the Leviathan, ex-hero of the Mandalorian Wars, and later commander of the entire Sith fleet, bombed Telos IV using his codes to bypass their scanners. Eluding Republic defense systems, Sith bombers were able to infiltrate into the Republic docks before any warning could be raised. In moments, half the Republic fleet docked at Telos was destroyed. This was only the vanguard of Karath's attack, and he soon arrived with the bulk of his fleet, the Leviathan at its head.

The resulting attack was devastating. While some were able to escape on in-system shuttles, millions were killed, and more died soon after due to medical supply shortages. A nearby Republic task force in which Carth Onasi was stationed arrived in the system shortly after, but by then it was too late. They were left to pick up the witnesses aboard the drifting vessels Karath and Malak let survive so that their testimony would intimidate those who would stand against the Sith Empire.

Aftermath
The attack left Telos's environment devastated. The atmosphere degraded into acidic vapor and all plant and animal life on the surface was obliterated. After the end of the Jedi Civil War, the Republic decided to use Telos for a test to see if a restoration effort could be mounted on the Outer Rim to reclaim the other war-torn planets, and financed the Telosian Restoration Project and construction of Citadel Station to purify the atmosphere and rebuild the ecosystem.

Republic soldier Carth Onasi was one of many left emotionally scarred by the attack. His wife dead and son missing, Carth swore revenge on Karath, who had been not only a hero to him, but his personal mentor. He came to fulfill this vow three years later on the bridge of the Leviathan.

In 3,951 BBY, Kreia predicted of Telos that "complacent and peaceful, it shall forget the time that Saul Karath orbited it and brought fire to its skies."

Behind the scenes
In Star Wars: Knights of the Old Republic II: The Sith Lords, a conversation between Jedi Masters Atris and Kreia pictures Revan instead of Malak as the one who gave the order to destroy Telos.

This content was cut supposedly to avoid confusion. While it is possible Kreia was just guessing the order came from Revan himself, that somewhat concerns Revan's romance with Carth for the non-canon female gender option, as it would make her responsible for the destruction of his homeworld and all that came of it.