There are ten Deckard's Journal locations to collect in the open world in Diablo 4 Season 15.
Collecting all 10 of Deckard's Journals unlocks Deckard's Memory and allows the player to visit Tristram in Diablo 4 Season 15. This location is an homage to the original location in Diablo II, where players would visit to rescue Deckard Cain early on in the game. Opening this location is a prerequisite to opening the mystery portal and acquiring the Hellfire Torch item.