After you defeat the Lich at the end of Ferun Castle, a dark red crystal/rock appears that Ellia identifies as the source of the dark energy. Afterwards, Durand gets in touch with you to tell you that you found a great clue.
He reveals that the black rock was a piece of the Kagor Magic Stone that was used to open the Portal to Another World in the ancient times (a stone that was supposed to have shattered when the Portal was sealed). It was believed that the stone had lost its powers, but it responded to the energy from the other world at the end of the Faimon Volcano campaign and regained its power. Lycana is working on purifying the stone and the Council is trying to find all of the rest of the pieces. Durand then asks you to go to Mt. Runar, east of Ferun, to investigate more reports of monsters acting strangely because of the magic stone.
In Stage 7, Mt. Runar's boss does not bring two underlings. He can stack up Continuous Damage fast and you have to be very cautious about his passive which will refill his attack bar at an alarming rate against certain monsters like Raoq. As suggested by the community, you will need healers and anti debuff, preferably 3 healers/support and 1 MAX HP damage based monster.
The Boss's skills are: Spitting Poison [Attacks the enemies with poisonous breath and inflicts continuous damage for 3 turns on each enemy], Cursing Breath [Attacks all enemies and inflicts damage proportionate to the enemy's MAX HP], and Atrocity (Passive) [Increases your attack gauge by 25% every time you're attacked and your speed increases greatly if your HP is below 50%].