Hotarumaru was owned by Korezumi Aso the head of the Aso family from the early Nanbokucho period 13341392. After a battle where the sword was badly damaged from Korezumis 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