When Zael and the party approach the bottom of the Turret Basement, Count Arganan used Zael's Outsider powers to summon the Lazulis Outsider. Count Arganan then began slashing at the Lazulis Outsider so he can unleash the full power of it by angering the Outsider. However, this only caused the awakening of the sentinel beast and the party had to confront it in battle. Later on in the story line, in the battle against Sentinel Beast Atar, Calista gains the Spirit Attack "Ancient Summon" which allows her to summon Mitra. Ancient Summon can only be used once in the battle, however, it can deal a large amount of damage and causes Atar's flying ability to be disabled.
Mitra is quite a fearsome foe in battle. It starts out flying around the stage which causes the melee hitters have a hard time hitting it. It can shoot multiple projectiles that can inflict silence and has an attack that fires out a laser which nullifies magic circles. To attack Mitra, you must silence it's magic so it can land to the ground. A good way to silence it is by Counter Blocking it's projectiles or diffusing Mirania's Magic Circles. It is a good idea to command Mirania to use her Nature Magic on Mitra since Mitra's only weakness is Nature Magic. In Co-Op mode, Mitra is a boss that can be fought against during the battle with Atar after taking out almost half of Atar's health. Fighting against both Atar and Mitra at the same time makes this one of the most challenging boss battle in Co-Op.