Right after the last quest has been completed, a new one will appear - Infestation. Unfortunately, more Sorrow Fragments have made their way to the mind of the main character. A new portal will open - use it to travel to the new location.
You will reach a new location, where your goal will be to defeat two Sorrow Fragments. Each beast has 54 health points, and is quite a challenging enemy. You will not have to fight them both at once - the next creature will appear after several turns have passed. Moreover, there are special panels placed within the zone - after interacting with them, a Sorrow Fragment will be lured to a specific location, and when the panel is used again the creature will be completely neutralized. This can make your fight significantly easier, so be sure to lure the beasts to a suitable location and then start the neutralization procedure, in order to vaporize the enemy on the next turn.
Keep in mind, that at any moment during the fight you can flee using the - you will reach The Calm, where you can head over to e.g. Tranquillity, to renew your main statistics. After you return to the area of the fight, the Sorrow Fragments will be on the same level of health that you have left them on before you fled. By using the hit&run method, even a band that is not very well developed in combat should be able to win the fight without issues.
After everything is done, you will not gain any experience points, however, you can interact with each of the tanks, which will unlock further memories (you will learn more about the Changing God and gain 3 XP for each of the interactions). Moreover, you will discover, that the Sorrow has still got a hideout in the Calm, but you are unable to detect it at this point. The mission will end in a success.