You will likely need to get your power level up to 900. This should unlock the special mission on Io. Once completed, you will need to talk to Drifter to gain the quest In The Face of Darkness. The quest is very short, and will unlock the Umbral Decoder and your seasonal artefact. To get your power level up to 900, running playlist activities like strikes, crucible and gambit will frequently give you gear that is higher power than what you currently have. Keep the highest piece you get in each slot and you'll quickly reach 900 and up to the soft cap of 1000. If you have too many umbrals and they are clogging your postmaster, you can dismantle the ones on your character to make space for normal legendary items in your postmaster.