Oni masks are mysterious objects that may randomly spawn on the ship in Monstrum. They are not items, and cannot be interacted with or picked up. They do not cause any anomalous or hazardous effects, and should not be considered an indication of anything.
Currently, their significance (if any) is not yet known, as there is no mention of them in any of the in-game notes. At this time, they appear to be nothing more than harmless macabre decorations.
- A cluster of crates containing possibly hundreds of oni masks may sometimes spawn in the lower areas of the cargo hold. This explains where the singular oni masks that can spawn in the upper levels come from, and suggests that they may simply be a curious export that the Hisa Maru was transporting.
- Whether or not any oni masks spawn has nothing to do with what monster spawns for that playthrough, nor does it affect their "stats" or abilities.
- As mentioned in the notes ,the ship was departing from Hong Kong, where the masks may have been picked up.
- They may be a nod to a new monster but it is highly unlikely.