At some time before 1906 he buried himself beneath San Francisco to enter a healing trance. When he awoke he found himself trapped, as a building had been built on top of him. Over time he went mad from hunger. He was only able to escape in 1906 when an earthquake rocked San Francisco.
When Samantha Jones and the Eighth Doctor were in San Francisco in 1997 he attacked Sam, feeding on her and practically chewing into her neck. Samantha Jones later killed Harold with a syringe of vampire repellent. (PROSE: Vampire Science)