Malvernis is a secret boss, although there is no quest given for fighting it, nor is defeating him is a requirement for the Tower Sweeper achievement. It initially takes on the form of a humanoid mage, with a long black robe with white trim, and a black cowl which obscures its face. It also appears to use Blood Magic and Rock Armor in the encounter.

In order to challenge it, the player must acquire the Warm Orb from a chest on a long platform inside the Carta hideout before the gateway to the fight with Rhatigan (where a Griffon statue that allows returning to Kirkwall sits).

It is highly recommended to be at a high level, for this fight is very difficult for weaker characters.

The battle begins with a black hunched-up entity spawning in the arena grounds, which begins summoning a host of 'Ghostly Archers' onto the platforms surrounding the arena. A good strategy is to rush one platform as soon as you activate the Obelisk and quickly wipe all the archers out with AoE attacks. Then, if necessary, you can move the party to the stairway leading up to the platform and close the door. The archers from the other side will not be able to hit you. That will give your party the opportunity to heal and reset the cooldowns before attacking the other side. If you take too much damage running across the arena, it's possible to position your party right next to the stairway wall on the other side, where it will be out of line-of-sight as well.

Once the Ghostly Archers have been defeated, Malvernis will unfurl and begin attacking. From now until the next stage he will summon Ghostly Skirmishers to attack you. He keeps summoning waves of three Skirmishers each until he is reduced to 3/4 health for him to change into the dragon (see below). If you don't have the damage output to rip through them immediately, a good strategy would be to have a ranged character kite Malvernis while the rest of the party deals with the skeletons. Make sure that that character is the only one getting aggro on Malvernis, and never stand still for more than a couple of seconds. Most of his attacks are AoE, and getting caught right in the middle can frequently kill even a tank in one shot. Also, when Malvernis teleports, he can sometimes switch aggro to the character closest to him. Generally it's a good idea to keep that character in motion as well until you figure out who he's targeting.

An alternate strategy is to move all range fighters to the rafters when he changes and position 2 near the trap trigger, keeping him near the spikes will net you at least 20% of his health with each hit. in 4 or 5 uses he will be defeated but if you do not finish the demons he summons off the battle won't end and his spells will continue.

When his health reaches around 3/4 of the bar, Malvernis will transform into a Ghost Dragon. This is now the same as battling a High Dragon. Defeat him using the same tactics.