Originating from a rich family, the siblings, Jarien and Sothos, were initially members of the Veiled Blade mercenary company even though they were not popular with their comrades. They were intolerant believers in the way of the Scarlet Crusade and aspired to knighthood within the organization, deciding to officially join them after the mercenary band was disbanded. They took the Left Piece of Lord Valthalak's Amulet with them and traveled to Stratholme, meeting Grand Crusader Dathrohan in his chamber. Unfortunately for the siblings, Sothos failed the final initiation rites and despite Jarien's succeeding and trying to defend her brother, both were slain on the spot by Dathrohan himself. As their souls were twisted further by their possession of Lord Valthalak's amulet, they still haunt the place.
Their ghosts can be summoned by igniting the Brazier of Beckoning within the Grand Crusader's chamber where they died - but be warned, of their former mercenary company, they are the hardest to kill.
Sothos and Jarien are undead warriors who seem to have split their class abilities between them. The brother, Sothos, has a shield and is the bad guy: he will [Fear], [Charge] around (apparently random) and do a very painful AoE[Shadow Bolt]. Jarien, the sister, is melee only and not hard to tank; her only special ability is a 1k dmg [Cleave] that hits everyone in the room. A good strategy is to kill Sothos first and have Jarien offtanked as he is the one that does the most burst damage. After he is dead, his spirit will help the fight against Jarien, but don't count on him to take too much damage as he will go down a lot quicker to her than he did for you.
Shadow Protection buffs or potions will help, as will a good Mana Potion for the casters. If you have a priest in your party, you can [Mind Control] the Crimson Inquisitors on the way in for a buff called "Shadow Barrier" that reduces Shadow damage taken by 50 for 10 min. Sothos and Jarien are also undead, so stock up on [Stratholme Holy Water] before the fight to make it a little easier for everyone.