The Scarlet Throne where Grand Crusader Dathrohan resides in Stratholme contains more than just the lies and madness behind the zealotry of the Scarlet Crusade but the restless souls of those who have failed the Crusade as well. Jarien and Sothos, two siblings who aspired to that order, now haunt the halls of the Scarlet Bastion, their spirits corrupted forever by the piece of the Lord Valthalak amulet held within their possession. They 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, the Veiled Blade, 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.