Warhammer 40,000: Battlesector

Chainswords are carried by Assault Squads, Lieutenant Carleon, Brother Aeuturo/Sanguinary Priests, Sister Verity and Death Company. As a melee weapon, they must be used as a charge attack or from a directly adjacent tile to the enemy. If you incorporate any of these units into your army, and use the Chainsword when possible, you should earn this very early on, maybe even in the Tutorial. It's random chance when you crit, but for each point of momentum you gain +0.4%, each mission survived +1% and some skills can boost it temporarily, if you're having trouble