While there isn't a specific item called "antiserum" in the game, "Deep Story" is a dedicated gameplay mode that allows players to unlock the "deeper" secrets of the plot.
Deep Story is one of three gameplay modes (alongside Casual and Another Story). It is essential for players wanting to uncover the dark origins of the RFA (Rika's Fundraising Association) and the Mint Eye cult. antiserum
Unlike Casual Story, which is more lighthearted, Deep Story focuses heavily on the overarching mystery, including 707's tragic past and his twin brother, Saeran (Unknown). While there isn't a specific item called "antiserum"