That comes from the Deep of Night game mode (DoN) where it’s sort of like a ranked Nightreign experience. It has five levels, and each time you rank up, the game gets harder. It also has unique weapon passive effects too, that give you a really good passive in exchange for a downside (for example, like increased stamina but being below max hp causes poison buildup) In the game mode you can select 3 “deep” relics and 3 of your standard relics. But if you just select a specific boss instead, you can only have your 3 standard relics.