Eerie Engrams are Festival of the Lost's newest addition to Destiny 2 and give players the opportunity to collect not only FOTL weapons like the Jurassic Green Pulse Rifle or Mechabre Sniper Rifle; these new engrams can also be focused with Auntie Eva to gather Exotic armor pieces.
For a chance to get these new Eerie Engrams, players must complete the returning Haunted Sectors from last year's event as well as the new tougher Legend Haunted Sectors. The latter of the two will provide an increased drop chance to incentivize players to dip their toes into the deep end of the challenge pool.
These can then be brought back to Eva to be focused into either a specific FOTL weapon or one of five types of engrams: random FOTL weapons, Exotic Helmets, Exotic Arms, Exotics Chests, or Exotic Legs. Each of these engrams has its associated costs, so make sure you're running the event enough to gather the Candy you'll need to collect them.
You can read over the entire Destiny 2 Eerie Engrams Loot Pool by following the link.