Lambent is a word used in either Gears of War or Gears of War 2 in order to show that something has been heavily exposed to a large or good amount of imulsion. In the Gears of War series, a couple of the Locust you face in the series are exposed to Lambent, and when they are killed, they explode.
 Gears of War
In the first Gears of War, the only Lambent monsters you will see are the wretches during Act 3. When you kill them, either shoot or melee it, they will always explode. According to your difficulty, such as normal, hardcore, etc. the damage will change heavily from barely any damage to half of your life in that one small explosion.
 Gears of War 2
In the second installment of Gears of War, Gears of War 2, you no longer see the Lambent Wretches. During Act 4, you will notice the rest of Delta Squad pointing out that the Locust are fighting each other. You will also notice the Locust Queen talking about a group of people or things being inferior. And she is not talking about the humans, but the Lambent Locust. Also during this, you will notice the Locust in the back round fighting the Lambent Locust. If you have a Longshot on you, you may be able to get a couple of snipes or nice head-shots, helping you with the achievement, Seriously 2.0.
The only other time that you will notice a Lambent creature is the Brumak in the end of the game. After riding it for a whole chapter and destroying underground pillar in order to make an opening for Ravens to pick you up. But after the Brumak unintentionally steps in a large amount of Imulsion, it is turned into a very disgusting looking thing that you must kill in order to sink Jacinto. This is because the explosion would be as large, or even larger than the Lightmass Bomb, which was knocked into a pool of imulsion by the Brumak along with the first Raven that attempted to pick you up.
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