- "The evening sky was alive with demons, the streets flooded with creatures."
- — Albireo —
Monster Invasion (モンスター街侵入) is an event that occurs every few months in certain Root Towns in The World. During the event, the protective shield around the Root Town is dropped, allowing various monsters to invade.
The event involved a swarm of monsters invading a Root Town like Mac Anu. Several waves of monsters, all of various levels, would sweep into the town, attacking any player they came across. In order to complete the event players had to band together to fight them off, eventually facing off against a powerful boss monster, usually a unique monster created by a CC Corp Administrator. The party responsible for defeating the boss of the event were rewarded with rare items.
With Lycoris's encouragement, Albireo took part in a monster invasion of Mac Anu. Cutting through waves of low-level monsters, he finally located the boss, a gigantic demon. With its immense power, the demon easily destroyed the low-level characters that had gathered around it, and Albireo was forced to fight it by himself. Though the monster nearly killed him, Albireo was eventually able to defeat it when other high-level characters arrived, including Orca. Having dealt the most damage Albireo was rewarded with a chest, containing an rae.cyl file for Lycoris.