Players will need to complete the Isaac Storyline in Lords of the Fallen to get this. To do some make sure to get the Flayed Skin from Abandoned Redcopse, which is to the right and down a side pathway after you face the Lightreaper and Otto in the tutorial boss battle.
From there, you need to access three memories:

• One at the entrance to Calrath City as you arrive from Fitzroy Gorge
• Two in Fief if Winter’s Chill. One in the ruin with the Vestige, and another after the Lightreaper Arena

After that, go to the Path of Devotion, and access a floating platform near the Sin Piercer archer on her dirt mound and her guard dogs. Open the Umbral pathway and go across to find another memory, which causes a fight with Paladin Isaac’s umbral ghost.

With Isaac defeated, get this Umbral skin, and return to the place you picked up the Flayed Skin in the Abandoned Redcopse tutorial boss area. This open up the way to get Isaac’s gear. From there, progress the game and fight the Lightreaper in Upper Calrath. Make sure to lose to it and respawn, as that will allow you to summon Paladin Isaac for the fight. Killing the Lightreaper while having summoned Isaac will award you the trophy.

For a more detailed guide, feel free to check out our Paladin Isaac questline.