This sounds like it was designed for Kōhaku Narukami and his Geist weakness Weeping Yurei. Although an annoying weakness if drawn at the wrong moment, you can pretty much control when the thing attacks you with a Kōhaku deck, and assuming you were investigating anyway Oculus Mortuum helps you getting the clue and at the same time stinging the little hit-and-runner. I see this as a true blessing regarding its keywords and the fact that the last damage will interrupt and negate the attack.
Soutien. Main
Objet. Outil. Occulte.
Coût: 3.
Utilisations (0 preuve).
Après que vous avez scellé, annulé ou ignoré un pion Chaos, inclinez l'Oculus Mortuum : placez 1 preuve dessus.
Quand un ennemi attaque un investigateur dans votre lieu, dépensez 1 preuve et inclinez l'Oculus Mortuum : découvrez 1 indice dans votre lieu. Si cet ennemi a le trait Geist, infligez-lui 1 dégât.

FAQs
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Reviews
I can only assume the TDC campaign box has some kind of Geist enemies in it that can be damaged by this, but it's straight up bad in runs of Carcosa or Circle Undone, which have Geists that resist most types of damage. Maybe particularly bad in TCU, where the game practically expects you to cancel certain treacheries to balance the game.