From Destinypedia, the Destiny wiki
Polarized Fractaline are a consumable only available during Season of Dawn and owners of the SoD Season Pass used to upgrade the Resonance Rank. You usually get Fractaline from The Sundial Arena, Bounties for Saint-14, and Season Pass rewards, but later on you will be able to collect Fractaline from Gambit, Crucible, Vanguard Strikes, and Generated Fractaline Harvests from The Tower Obelisk.
A community Co-Op event known as the Empyrean Restoration Effort or the Empyrean Foundation used up more that 7 billion Fractaline to power the Sundial's core, now inside the Tower Obelisk. This allowed a new area known as "The Lighthouse"* to be built using the Season of Dawn Artifact at the Caloris Spires. The Empyrean Foundation ended on February 26, 2020.