Festival of the Lost 2024 offers one of the most sought-after cosmetic items in Destiny 2: a Lost Memento. The memento comes with an all-black shader (even better than Superblack), and one of the ways to get it is by completing a secret Twilight triumph.
You can find the triumph on the second page of the General tab inside the Episode: Revenant triumph section. Unfortunately, the objectives “???,” “?????,” and “???????” don’t really tell much and will leave you scratching your head almost as much as the Festival of the Lost weapons being pink, for some reason.