Caladrius is the leader of the slavers behind the Unrest in the Alienage that The Warden is sent to investigate. He has documents proving Loghain's complicity in the slave trade that The Warden must obtain. The Warden can negotiate with Caladrius, allowing him to live, or choose to fight him. The negotiation is 100 Sovereigns from The Warden in exchange for a letter from him implicating Loghain, and permission from the Warden to take the remaining slaves when he leaves. Most companions will not approve of this though. Alistair says "I feel dirty... We're not considering this are we?" Zevran tells the Warden to "Look these Elves in the eyes before selling them off." Morrigan, by comparison, will state that it is "a reasonable enough starting offer." Once defeated, he will offer to use blood magic to sacrifice all of the slaves around him, improving the Warden's health. Wynne (if in the party at the time) will also warn the Warden that Caladrius is talking about Blood Magic. Most characters disapprove of the sacrifice, which grants the Warden +1 Constitution. However, Sten states that this is a prudent choice, and Morrigan disapproves if you decline the offer. Alternatively you can loot the Robes of the Magister Lords from his dead body.

See Unrest in the Alienage for more details.