From their website:
The forces of Everblight do not thrive due to numerical superiority or fortified holdings but rather due to their adaptability. Everblight’s spell martyrs are a gruesome example of this ruthless cunning. These blighted Nyss are imbued with the dragon’s blighted energy, allowing warlocks to tap into their essence to unleash powerful spells. This overwhelming flow of arcane energy consumes the spell martyr just as it consumes the bodies of Everblight’s enemies.