Niv-Mizzet Reborn

Legendary Creature — Dragon Avatar

Flying

When Niv-Mizzet enters, reveal the top ten cards of your library. For each color pair, choose a card that's exactly those colors from among them. Put the chosen cards into your hand and the rest on the bottom of your library in a random order.

A "color pair" is exactly two colors. There are ten color pairs in Magic: white-blue, white-black, blue-black, blue-red, black-red, black-green, red-green, red-white, green-white, and green-blue.
If the top ten cards of your library don't contain all ten color pairs, you choose as many cards as you can and put those cards into your hand.