Sangrias Stillblade is a Blood Knight sent to test new members of the order in mortal combat. He is summoned by lighting a brazier in a small cave on the Isle of Tribulations.
He bears the same last name of Lonika Stillblade but it's unlikely they are related.
Stillblade is sent by Knight-Lord Bloodvalor to kill the newest initiate (player) as a test into getting into the order. If he succeeds, then you weren't meant to be part of the brotherhood; if he fails, a few initiates are sent to gather his body and place it on the second floor of the Silvermoon City Inn, where he remains dead, until you can revive him.