The Alien Monster is a solo mission you can take after the Wind Dragon is slayed in the Human Path. Once you pick the request up from Thanos head down to the Septem Region and take the turn that comes just before the hunting village Garcia lives in. The entrance to Algandars Castle will be in the side of a large rock. When Jack voices his suspicions of the group of mages, Ardoph says that they are not bad people and that it was Void that asked them to decipher the seal of a certain crystal. They created a seal and were doing their research when the seal broke and the demon appeared.