From Halopedia, the Halo wiki
An energy cutlass, or painglass saber, is a Covenant melee weapon primarily used by the Kig-Yar for light skirmishes in close-quarter situations. It is an energized cutlass forged using a modified energy projector with a Subanese crystal shard that has been chemically treated and sharpened into a blade.
This bladed weapon is composed from a crystal shard from Suban and a complex arrangement of chemicals, plasma, and alloys. Just like the shattering of a fired Needler round, the blade of a painglass saber can be made to explode when it is embedded in soft flesh, making it a terrifying and cruel weapon. Before detonation, the blade will begin to pulse with a rosy red color.
Referred to as chiss-teh by the Kig-Yar, these crystal blades are utilized by Kig-Yar soldiers as well as pirates, as they make ideal weapons for boarding actions on hostile ships or in melee engagements in confined spaces. Both Avery Junior Johnson and Nolan Byrne survived an encounter with these deadly arms upon making first contact with the Kig-Yar of Minor Transgression in 2525. Despite being pierced in his left thigh by a cutlass, Nolan Byrne managed to pull the blade out and even kill his alien attacker with its own weapon.
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