Question

How do you get shiny legendarys?

Answer 1

You soft reset.

Step 1. Stand in front of the legendary, but don't interact with it.
Step 2. Save the game.
Step 3. Interact with the legendary. If it is shiny, catch it. If it is not shiny, go on to Step 4.
Step 4. Turn off the game without saving.
Step 5. Turn your game back on, and repeat Step 3.

It will take a loooong time for the legendary to be shiny. It's all based on chance/luck.

Answer 2

Same way you do with any other Pokemon - 1/8192 chance. Except, if it's not, you have to restart your game entirely.

Source: Experience and other answers here.

Comments

is it possible when you say on newmoon island with Darkrai can it be shiny?

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Well, if you cheat for the item yeah. Unless they Shiny locked it which is plausible.