A hag, of course. Black Annis (CR6), to be specific.
Why?
Maddening whispers, fog cloud Annis ability, terrifying combat ability.
Add 5 levels of Druid (for harassing spells, animal companion, wildshape ability)... "I though I heard someone speaking, but all I saw was a crow, perched in a withered tree."
Add 1 level of a rogue (sneak for additional damage, improved stealth abilities).
Total CR: 12 + 2 (Wendigo) = 14.
Remember to add some magic items capable of either turning the hag ethereal or foiling detection spells. A scroll or a ring with two or three Dispel Magic, Greater... and a few pet Stick Golems.
Regards,
Ruemere
PS. I guess my inspiration is obvious. In order to increase sheer deadliness of the encounter (or to scare the players more), add a few natural hazards, like colonies of slimes and oozes.