Hotarumaru

Hotarumaru was owned by Korezumi Aso, the head of the Aso family from the early Nanbokucho period (1334~1392). After a battle where the sword was badly damaged from Korezumi's mowing movements, Korezumi rested at the foot of Mt. Aso, falling asleep out of exhaustion. He had a strange dream where a swarm of fireflies (hotaru) gathered onto his sword, making it glow. The next morning, much to his confusion, the sword appeared to be fully repaired. The sword, later named “Hotarumaru”, was kept in the Aso shrine as a family treasure.

(Source: Touken Ranbu Wiki)