Diablo 4 Season 15 is stuffed full of secrets years in the making, and despite the season having launched two weeks ago, players keep discovering new ones.  Season 15, Hell’s Legacy, is a major nostalgia trip for Diablo fans. It adds new dungeons, boss fights, and items from previous Diablo games to Diablo 4, as players team up with Deckard Cain to defeat echoes of the Prime Evils and harness some of their iconic powers. But those references to previous…