Watch the video to see where to go to unlock Ending B.

First, go to Sorcerer's Academy and talk to the Hooded Person. Then, talk to the person again at Crimson Forest. Next, go to the Land of Origin and approach the sealed door. After that, head to Central Stratum - Central City and talk to the Hooded Person again by the flower. Finally, return to Lilia at the Land of Origin.

You will unlock Lilia's Blighted Ring. Equip it and fight the final boss again to unlock Ending B.

Note: Before you can complete this questline, you also need to follow Levy's quest. When you meet her in the Land of Origin, she will give you the Stele of the Land of Origin, allowing you to start talking to the hooded person next to the flower in the Central Stratum.

How to unlock ENDING B
https://www.youtube.com/watch?v=Gh12B7mz5gw