Mission 21 through Mission 26 are all tasks that are given to you by the Menhirrim inside Taejin’s Tower. They are required to be completed in order to progress the story of the game and need to be completed in order to navigate through the various tiers of the tower.
The easiest way to complete these Missions is to follow along with the Taejin’s Tower walkthrough page.
This particular Mission does have prerequisite requirements in the sense that you can only reach this Mission by activating the Menhirrim on the Second Tier which are involve in the following:
The Menhirrim that initiates this Mission is located on the Fifth Tier, just like the previous Mission, but you will need to first reach the Sixth Tier. Refer to the Taejin’s Tower to find the complete set of steps for initiating this Mission.
Pulling open the mini-map reveals that the mark for this Mission is just in the next room over but there is no connecting path just yet. The destination marker shows you where you need to go.
Exit this room out into the hallway and head left. There are no Treasure Spheres in this area. When you reach the next room, you will be ambushed by a new group of enemies that look like bats; they are called Chonchons. While they can deal out a fair bit of damage if they focus their attacks, they generally go down very easily.
Stand in the illuminated red circle in this room to activate the Menhirrim. This process will cause the structure of Taejin’s Tower to switch around allowing the main tower elevator to access more rooms. More importantly, it allows you to reach the mark for Mission 25.
Exit the room and travel to the right, taking the second door on your right, and head towards the ramp. Before you travel down it, open the nearby Treasure Sphere to obtain the 
Unsetting Sun weapon for Snow.
Travel down the ramp, back to the tier below, and the mark, Vetala, can be found floating around at the bottom.
Class: B
HP: 480,249
Vetala will cast “Inertial Barrier” at the beginning of the fight which will make it immune to physical damage. 
Commandos will be able to get around this limitation by attacking it with Ruin, but even when using magic-based attacks, they will only hit for 50% of their normal damage.
The idea behind this fight is to get Vetala to its stagger point and then switch over to physical attacks to bring it down quickly. At this level, you will likely need to stagger it twice.
Using the navigation links below, you can also quickly jump back to the Chapter 11 strategy guide, straight to the next mission, or back to the main Mission index menu:
Cobaltite
Bomb Core
