The Crusader is a quest in Epilogue.
Open a map that's in in one of the corner regions of the Atlas (Haewark Hamlet, Lex Ejoris, New Vastir, Lira Arthain). The first time you do, Baran will appear and taunt the player, adding his influence within the map region. Repeat this two more times, then talk to Kirac. He will show the location of the Citadel in the region, then talk to Zana to open six portals into Baran's Citadel. After defeating the map boss, go into the portal and defeat Baran, the Crusader.
To spawn the Conqueror again, open a map in a region with one Watchstone slotted with a map of the appropriate tier (as long as their corresponding Watchstone hasn't been found yet for that region). Find their Citadel again by running maps within the region they're influencing two more times, then repeat the process of entering their Citadel and defeating them. Repeat this process with another Watchstone. After the fifth time, the Conqueror will be defeated, completing the quest. They will not appear again until you have challenged Sirus.