The Demonic Remains quest is pretty simple and requires you to perform a Challenge Rift. The following guide will cover what you need to do in the quest for the Demonic Realm in Diablo Immortal.
Diablo Immortal Demonic Remains Quest
Once you are done with the Zoltun Kulle Library questline, head back to Westmarch. Deckard Cain will introduce you to Rayek, who is also a close friend of Charsi.
Rayek will ask you to track down and hunt down a Chaos Herald as part of the Demonic Remains mission. To hunt him down, you must enter and complete a Challenge Rift on the third difficulty level.
Make your way to the Rift of Challenge entrance inside the Palace Courtyard in Westmarch. Click on the Challenge Rift statue and set the difficulty level to 3. Choose whether you want to go alone or with friends and click Enter.
Just like the rest of the Rift Challenges, you must defeat the Rift Guardian before an on-screen timer runs out. Start mowing down enemies as soon as you spread out inside the Rift.
When you reach the end of the corridor, you will encounter Chaos Herald Pyl. You have to defeat him to complete the Rift. He just looks scary and doesn't do much damage. You should find this boss fight relatively easy.
After defeating Chaos Herald Pyl, you will receive Pyl's Flowing Chaos which you must return to Rayek. Use the portal to return to Westmarch. Go to Rayek, run out of his dialogues and he will ask you if you have collected and used the Elliquary (the Elliquary section will be unlocked within your menu).
To use the Elliquary, you will need to select demonic remains to seal. For this instance, you will be using Pyl's Flowing Chaos to seal, which will give us an additional bonus that can only be activated in Challenge Rifts.