As it turns out, there was a bug in Diablo 4 Season 15 that prevented players from finishing the main questline. Mercifully, the issue has been rectified in the latest patch, though there are other problems players hope Blizzard will look into.
The topline change in the update focuses on players getting access to a Splinter of Prime Evil before they're supposed to. Doing so stops you from finishing the central quest, because the Prime Evil powers are meant to be part of your reward.
The glitch has apparently been rectified, but anyone who's already gotten it will need to use Blizzard's workaround to sort out their character, since quest progress isn't affected. "This change prevents this bug from happening but does not reset quest progress on characters who are already bugged," reads a developer's note.
"To allow affected characters access to Prime Evil Powers, create a new character in the same partition (Normal/Hardcore and Normal/SSF) and complete the seasonal questline," the post continues. "This unlocks Prime Evil Powers for all characters in that partition."
Not ideal, but at least there's a fix of some description. Another bugbear where four affixes couldn't be rerolled using the Horadric Cube has been remedied as well - sadly, the wasted resources can't be returned, but at least it's sorted.
While these are certainly helpful alterations, they don't stack splinters, a mechanic players have been requesting for several weeks now. "Splinters not stacking is truly the dumbest decision of the season," writes one Redditor, summing up a theme across the comments responding to this update.
We'll see if Blizzard decides to address that problem in the coming weeks. For now, at least you can definitely complete the current questline without extra hassle.
