Seth stole the keycard and you are stuck inside the dark crypt.

Use the hotspot to try to find something that helps you.

Take the viscous mass from the ground. You can see it as the only item in your inventory.

It is indeed the only item you can use in the dark crypt. Try to find some place to use it on.

Did you notice you can go far left? All the way to the entrance you originally came in.

Go all the way left until you find the "Plant?".

Click the plant, it drop something burning... Use your mystery object from the inventory on the burning object on the ground.

Go talk to Clayton.