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. Sam later killed Harold with a syringe of vampire repellent. (PROSE: Vampire Science)