The monster should be a challenge and withstand a fireball (at least theoretically). So it should have at least 6 hit dice. It's not particularly hard to hit, but all the various mists seem to be immune to anything less than a magic weapon +1. Let's keep that and intensify: the mist is immune to any weapon attack. But they are vulnerable to any sort of fire. If hit by magical fire they must make an immediate moral check or withdraw. The mist doesn't do conventional damage. It is deadly cold and sucks your life energy away. If you remain conscious, the betrayal will happen. A random member of the group is charmed to attack his comrades (save vs. magic; save can be repeated each round). Effects wear off, as soon as the group leaves the mist.
That's what my mist of betrayal looks like. It's more of a natural hazard than a monster. But still it can be "killed".
Mist of betrayal: HD 6, AC 14, Speed: half that of a walking human, Morale: 6, Damage: special (see below), Immune to all weapon attacks, vulnerable to fire (if attacked with magical fire must pass morale check or flee), AL: Chaotic
- Hard to perceive: Succeed at a bushcraft check to notice the mist before it solidifies.
- Cold, clamy mists: Save vs. poison each round inside the mist. On first failed save, you are knocked out. On second failed save, you die. If only knocked out, you regains consciousness one round after leaving the mist.
- Betrayal: A random party member who is not knocked out must save vs. magic or be charmed to attack his comrades. Save can be retried each round. Leaving the mist ends the effect.
- Leaving: Take a round to flee outside the mist. Do not attack during that round.