Currently, as of Minecraft 22w12a (1.19), Wardens are in the game. They are blind mobs that can sense vibrations.
If Requiem is to add support for possessing Wardens, two new tags would be required: #requiem:blind_mobs and #requiem:sense_vibrations (names subject to change).
These tags would be empty by default until 1.19, at which point the entity type minecraft:warden should be added to both tags.

Anyone possessing a mob that's part of the tag #requiem:blind_mobs would get a constant Blindness effect until they stop possessing.

Anyone possessing a mob that's part of the tag #requiem:sense_vibrations would get notifications on their view when the possessed entity detects a vibration (just like a Warden).

Datapacks can override the above tags to add or remove entity types that will cause the possessor to get the Blindness effect and/or receive vibrations when being possessed.

You would need a tag to say that vibrations can be sensed by the player as well.