The player must complete the Four Horses of the Apocalypse challenges. After taming all four Horses of the Apocalypse and killing one of every type of undead with Death's ability, the player will unlock the Blood Pacts. Also, the player will receive a Blood Pact for Death once they complete the game, even if they already had one. (This means the player can have two Blood Pacts for Death).
These pacts work much like Horse Deeds. The player simply selects one of the pacts from the inventory to activate the associated horse. After doing so, whistling will call the selected horse to the player.