Based on S.D. Locke's Proposal - SCP-001: When Day Breaks

3891 days ago, Equestria and the world itself changed. Warm sunlight shined on the land, and it seemed like it was a normal day. But as the warm sun beat down on the land... billions of ponies and creatures died, and the sky turned red like blood.

Rarity is an ex-employee of the SCP Foundation. When the day broke, the foundation fell. Only one location remains; Site-19. It is Rarity's new home underground, where her status is null but she is safe and... alive.