How do you complete Darkrai event?

It's impossible to complete a "Darkrai event" without more context. Please provide more details about what you're referring to.

Here are some things that could help me understand:

* What game are you playing? Is it a Pokémon game? Which one?

* What is the event about? Is it a specific event in the game?

* What have you already done? Where are you stuck?

Once I have more information, I can help you figure out how to complete the Darkrai event.