Lay is training to become an angel of death. Meaning he was human, a qualified human, and then died, giving him the option of becoming an angel of death on EXO.

He completes his angel training and is granted his wings. Wings are given after training, but before the Final Test. Every angel, whether they are training for being an angel of death or not, does a final test, but the trainees don’t know about this test. If you pass the test, you keep the wings and you don't find out that you even went through the test. Later on, when you are a complete angel you are told about the test that you did and passed with flying colours.

Those that fail however, their wings are taken away and they are sent to the Spirit World, where the contact between these two worlds is cut off. They are forbidden to tell anyone about this test and their mouths are sealed from leaking out any information regarding this matter. It isn’t a punishment, they would have gone to the Spirit World anyways, if they had died and chosen not to become an angel

Lay is happy that he completed his training and is now an angel of death (or so he thinks), and is overjoyed at the thought of joining his friends as a full time angel.

But he always wondered what happened to Suho, one of his friends. He had never seen Suho after he had been given his wings. He seemed to have vanished. He didn’t know why or how this happened, but the official angels seemed to know.

Lay’s test is approaching…Does he pass and gain permission to stay on EXO? Or is he banished and sent to the Spirit World?