Updated.
For skills, how about: Knowledge (the planes) 8, Listen 8, Search 8, Sense Motive 8, Spot 8?
For feats, how about Ability Focus (mesmerizing lights), Weapon Finesse?
For the languages, we could borrow from another inhabitant of their home plane, the trilloch, and go with "Grave watchers cannot speak, but they understand all spoken languages."
Also, we really need to add this ability, common among Negative Energy Plane creatures:
Negative Energy Affinity (Ex): The grave watcher is affected by cure spells and inflict spells as if it were an undead creature.
CR 4? They are hard to hit and have hefty resistances, plus can render those that see them nearly helpless.