I've been playing around with haunts for a Pathfinder adventure I've been asked to write. Man, they can get unbalanced fast, and need to be sharply reined in to serve the story being told. Here's one that is legitimate by the rules, but can kill just about any low-level character without recourse. I absolutely promise I'm not using this one.

 Murderous Word CR 6

XP 2,400

CE haunt (a room 30 ft. square)

Caster Level 6th

Notice Perception DC 15 (to hear a short intake of breath, as though someone were about to speak)

hp 12; Trigger proximity (triggers on initiative rank 20 rather than 10); Weaknesses susceptible to sonic damage, tricked by Stealth;  Reset 1 week

Effect A random target in the area hears a word of indescribable malice and is struck by a power word kill spell.

Destruction The entire room must be magically silenced with sufficient silence spells to cover the room.