When Winter arrives, when it is a normal snowy day, you think you are fine, living a good life, but no. The bird arrives. The Roaring Bird. The prophecy stated that this is no normal bird. It is the Roaring Bird, and its mission is to freeze anything in sight, and turn the world into an endless, snowy Winter. There is nothing that can stop the Roaring Bird. (The joke is that Articuno with Snow Cloak and tera Ice and Choice Specs under Snow and Aurora Veil is immortal.)
Articuno's sprite found from Bulbapedia. Music from Toby Fox. The snow engine (which isn't the best) and the Roaring Knight engine are coded by me.
