The backstory told by Brom Bones was accurate. Filling in the gaps, it was the Friends that allowed his body to continue living, terrorizing Sleepy Hollow in search of a head. He killed several before Ichabod, but he never found one that suited him. He continues to terrorize the town, even if he's found his head, because his debt has not been repaid.
- Depending on your interpretation of the story, the one who has the deal with the Friends could be Brom Bones. He summoned (or was temporarily transformed into) the Horseman to destroy Ichabod. In the end, however, he couldn't go through with it, but still chased him out of town. Either way, Sleepy Hollow had the Friends' fingerprints all over it.