Kryll are a form of carniverous bat. Alone, they aren't much of a threat but in a swarm, they will quickly consume even large armies. For this reason, it was unsafe for Humans to leave the area around cities at night for almost the entire war (until after the Lightmass Offensive).
The only thing known to be effective against the Kryll has been Ultraviolet Radiation. This prevented Kryll from leaving the caves of the hollow during the day. The COG quickly mass produced UV lights to protect their cities from the Kryll at night. Some of these lights were weaponized for instances where missions had to take place far from cities at night. These weapons were almost exclusively seen mounted on vehicles because of the amount of energy that was required to power them.
During gameplay, it is important to stay out of the dark. Swarming Kryll are usually used as a sort of soft boundry to prevent players from getting to a certain area. Rather than outright killing them, they will be damaged quickly. If the player doesn't respond in time, they will end up dead. This makes Kryll an interesting gameplay mechanic where the landscape of the battlefield can be altered by activating or deactivating different lights.
