When you get there, search for the green circle that will apply damage over time whenever you stand in it.
Going to the Vanguard of the Three area will be crucial in getting the Chillheart Legendary Sword. Images in this article were taken from Luzzifus on Steam. Unlocking the secret Crate of Entertainment boss, and finishing it of course, will reward you with a Devotion Shrine and two really good, special legendary items that any collector will want.
The Crate of Entertainment secret quest is only available on the Ultimate Difficulty in Grim Dawn (Forgotten Gods) and there is no quest log entry for it. Since this whole quest is hidden, you won't be able to kill Crate of Entertainment just by going through the game- and you'll miss on a couple of great rewards. Crate of Entertainment is the latest secret boss which was added in the Forgotten Gods expansion.
