Defying Death

Many people have theorized what Sunny's act of DEFIANCE could be.

Some with bad theories about having to kill himself.... Where did you guys get that from? What's the point of getting stronger by destroying a part of yourself?

Some of them are quite interesting, like challenging his flaw.

But do you know the biggest challenge for the heir of death?

Bringing someone back to life

This could be Sunny's act of defiance. Defying the natural law of death, an absolute law. Think about it, the Shadow God can only curse someone not to die. We still don't know if it's ever brought anyone back to life. But let's assume not.

Why do I think this will be Sunny's act?

In chapter 2018, Sunny is pondering whether or not to kill Beastmaster:

[...]However, bringing her back from the dead seemed like a very troublesome task.[...]

This could be a hint of things to come in the future.

Of course, probably if Sunny manages to start reviving the dead he will have some restrictions. Like doing it before the person's Shadow is consumed by the Shadow Realm. And having a compatible body to receive that Shadow.

(understand this is a THEORY, it may or may not happen.)