Image: Rebel Wolves / Bandai NamcoNo screenshot of Blood Surge yet. This one shows Svartrau Cathedral (Rebel Wolves / Bandai Namco).
Blood Surge is a night-only Vampirism ability that makes a target burst from the inside through their own blood. You unlock it by drinking Xanthe's blood, so it is tied to her questline.
Find the Treasury Key, open the casket near Svartrau Cathedral and drink the Phial of Xanthe's Blood. The other route is beating Xanthe in The Cycle of Love and feeding on her. After that, spend a skill point at a road shrine, with enough Corruption, to learn each level.
Blood Surge detonates the target and splashes damage around them. It is a strong answer to groups at night and adds big burst damage in boss fights.
It is a vampire power, so it only works after dark when Coen is in his vampire state.