The dying sun

The Dying Sun is a key character in the main story, it's fire and warmth are slowly draining. This is why you collect 7 flames and bring it to the Sun to help it survive longer. At the end of the story, you must go to Sun to deliver the flames. There's a small cutscene with Sid being freed, saying how "you have no idea what you've just done." After this cutscene and ascending into the sky, you'll get a mask called the Sun Mask as a reward for completing the story and reaching the sun.