From Destinypedia, the Destiny wiki
The Sundial Spire is a hidden area on Mercury that houses Osiris and the Sundial. It is only accessible by players who purchased the Season Pass of Season of Dawn. It is the main area of the arena The Sundial. In the Quest An Impossible Task, players can use the Sundial to walk the Corridors of Time on a mission to save Saint-14 from the House of Rain. In the second part of that Quest, Players use the Sundial once more to stop Agioktis, Martyr Mind from draining Saint's Light. Later, after The Guardian had saved Saint from the Corridors of Time, Osiris invited the Guardian back into the Corridors saying that there 'are so many secrets that you haven't uncovered.' This led players to figure out the 21 different codes; 19 with lore, the 20th being an Emblem, and the 21st being a grave to The Guardian itself. A week later, the Corridors collapsed.