Mortal Shell 2 Seals quick answer: compare all four before you equip Slayer
Four-Seal comparison with the Slayer equip warning, current defensive-role guidance, and acquisition summaries: Mortal Shell 2 currently has four documented Seals, Untarnished, Vatra's, Infinite, and Slayer. Untarnished is the safest first-playthrough default because it keeps a held Guard fallback behind Perfect Guard, Vatra's shifts the button into Harden pressure, Infinite turns parry into the full defensive plan, and Slayer is the sidearm-focused outlier that permanently disables achievements once you equip it on that save.
Acquisition summaries are simple: Untarnished is your baseline, Vatra's comes from the Temple of Vatra region, Infinite is tied to Crypt of Martyrs progress, and Slayer is safe to collect but not safe to equip on an achievement run. If the question is which Seal to use, start Untarnished unless you already know you want Harden pressure, boss-timing parries, or a Slayer-only challenge file.