Sins of the Horadrim
Season Timeline
Seek Penance in the Seasonal Questline

All bonds that hold strong are eternally subject to the entropy of the universe; they crack and fragment, eventually unleashing whatever horrors lay within. Season 9: Sins of the Horadrim takes you to Cerrigar, where you’ll meet the Druid Bryona. Bryona must contend with decisions made by Donan—the Horadric Mage responsible for sealing away Astaroth upon his defeat—that now threaten to once again endanger her home and her people.
It’s your task to discover and purge the blood relics influenced with Astaroth’s blood before they fall into demonic hands.

To begin the The Embers Yet Smolder questline, find Bryona in Cerrigar, northwest of the Waypoint. You must have also completed Diablo IV’s base game campaign and created a new character on the Seasonal Realm.

During the Seasonal Questline, you’ll discover the secret to unlocking Horadric Strongrooms and delve further into these archives of this once sacred order. You’ll also gain access to the Horadric Focus in Cerrigar. This powerful device of the Horadrim is where you can check your Seasonal Reputation (Horadric Knowledge) and earn rewards from the Seasonal Reputation Track.

Horadric Knowledge can be earned on the Seasonal Realm by slaying Mummified Horadrim in Strongrooms, Elite Enemies and Bosses. Note that Lair Bosses also grant Horadric Knowlege, but you must unlock their respective Hoard upon their defeat to claim it.
Horadric Phials can be earned in Nightmare Dungeons, Nightmare Escalations, Horadric Strongrooms, and Lair Boss Horde Chests. These Phials can be used in exchange for Horadric Knowledge at the Lapidary, and to craft Horadric Jewels.
This Questline also grants you access to the Horadric Wayfinder, which allows you to find Horadric Strongrooms hidden within Nightmare Dungeons.