Lightsabers could deflect directed-energy weapons but a wide-beam phaser could kill the wielder of a lightsaber provided the beam was wide enough. They were dangerous to use due to the likelihood of the user cutting him/herself to pieces through improper use. The color of the crystal contained inside determined the color of the blade. The best crystals used in the crafting of light sabers were to be found in Klach D'Kel Brakt. As a matter of convention, the color of a lightsaber is that of its blade. When not in use, the hilt could be disguised as a flashlight. (Star Trek: False Vacuum: "Black Tornado")
In the Klingon Empire, lightsabers were outlawed at some time after the Battle of Klach D'Kel Brakt, whereas they were legal in both the Federation and the Interstellar Concordium. (Star Trek: False Vacuum: "Black Tornado")
It is unknown when exactly prior to 2271 the Klingons acquired lightsabers. In the Battle of Klach D'Kel Brakt, lightsabers were used to great effect by a cadre of elite Klingon warriors. Kor did not personally use a lightsaber however. (Star Trek: False Vacuum: "Black Tornado")
Captain Lewis, and Jono possessed these and were skilled in their use. The Borg Green Warriors also used their own version of the weapons. (Star Trek: Unity (fan film series): Final Order (Unity episode))
In 2415, a shipment of contraband lightsabers was seized from the House of Korgath, along a shipment of white kryptonite, near the Interstellar Concordium border. Also, both Ariane Binks and Dunames Lopez, among others, took second-hand lightsabers for themselves. (Star Trek: False Vacuum: "Black Tornado")
Ariane took it upon herself to redesign the hilt of her lightsaber, plus Janielle Potvin used that lightsaber as a light source when escorting the prisoners back to a safe area. Three lightsabers were also used in tracking down a wide range of criminals, such as Q'Naabian mobsters and Orion Syndicate gangs. (Star Trek: False Vacuum: "Pothole Omnium")
Although the majority of the details of the Battle of Klach D'Kel Brakt were left out of the episode, the DS9 episode Blood Oath established that Kor did not fight at Klach D'Kel Brakt with a lightsaber.