Vestige

The Vestige is one unique creature: a broad, creeping cloud of mist lit from within by a weak, flickering glow. It formed from the nightmares that every citizen of a cursed sleeping city dreamed as they died in their enchanted slumber.

It roams the city's heights and depths without rest, flowing over walls, bridges and open air alike, and homes in on any thinking mind within 1,000 feet. Visitors have a 20% chance every four hours of meeting it, moving or hiding; only an airtight seal keeps it out. It prefers to approach unseen, along roofs or under bridges.

A creeping dread announces it. Within 100 feet a chorus of moaning voices becomes audible, even through silence, and all in range save vs. spell at -4. Failing by 1-4 means -4 on every roll, attacks and checks included, while it stays that close; failing by more causes a full turn of mad flight.

Within 20 feet it attacks with 1d12 mist streamers per round, each able to strike a separate target for 2d6 as flesh dissolves. A thinking creature inside the cloud makes an Intelligence check each round or loses 1d4 Intelligence. Drained to zero, its mind joins the Vestige forever (friends can hear its voice in the din), recoverable only by a well-worded wish, while the husk melts at 2d6 hit points per round. Mindless beings, undead included, start melting at once, on top of streamer attacks. Survivors regain 1 Intelligence every 12 hours.

Cold, poison, death magic and charm, hold or sleep effects fail against it; fire and electricity do half damage. Only +4 or better weapons hurt it, dealing just their bonus. Protection from evil and similar wards bar it without a magic resistance roll, but it breaks through in 2d6 rounds. It cannot be turned.

It simply drifts through the demiplane that was once the city, voicing its anguish. It cannot leave; outside the city it would dissolve.

It feeds, in a sense, on minds, yet each one it takes becomes part of it and shares its torment forever.