I fought the Chaos, and the Chaos won

Killing someone is, after a certain point, a supreme waste of time. For creatures native to the plane, it's a simple matter of 25,000gp (at most), a high level priest and 10 minutes. For outsiders, killing them normally banishes them from the plane for a while, but there's nothing stopping them coming back later, or continuing their mission remotely.
The only way to really stop someone is with a temporal stasis spell. This locks them in place, and prevents them from being resurrected or summoned easily. Cast the spell on them while they're in a locked, timeless plane and it's very hard for outside forces to rescue them. The downside is that if temporal stasis is cast in a timeless plane, the creature retains their ability to think and move, although they are helpless to escape. However, this does give them time to plot their revenge against their captors, or just cut out the middle man and slowly go insane from rage and captivity.
It's not a pleasant fate, but if no better options present themselves it can be used to lock away the very worst (or best) that the planes have to offer. Over time, some of the Valley's worst have ended up in a prison deep within Rappan Athuk known as The Shade, The Oubliette, or sometimes just "Room 302"
==== Classification: Lhaksharut (Inevitable) ====
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* Name: Planar Enforcement Officer #10176182
* Crime: Overzealous in its duties. 10176182 was sent in to restore order after Muir's attempted coup. Deciding that the Valley lay on a fault line between the planes, it attempted to "clear the area", and managed do a large amount of property damage and kill or injure dozens before it was secured
==== Classification: Akvan (Div) ====
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* Name: Cataclysm
* Crime: Cataclysm is so named because it was the scourge of the plane of air, slaying djinn and raising them as ghuls in a prolonged rampage. A Djinn sorcerer managed to throw it off the plane - unfortunately it landed in the Valley, where it promptly began rampage once more. Killing it would allow it to return to the plane of air, so it was captured instead (at great loss of life)
==== Classification: Hive Queen (Formian) ====
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* Name: n/a
* Crime: Most formians exist away from civilization, but this queen 'claimed' the lands of the Valley (at the site of present-day Tsar). It dug elaborate tunnels (which would later become the core of the Tsar catacombs), destroying sections of the city and allowing easy passage for the horrible things lurking below up to the surface. It could not be reasoned with, and was eventually secured for its own safety as much as for Tsar's.
==== Classification: Maralith (Demon) ====
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* Name: Carraibhal
* Crime: Attempting to lead an army of demons to crush the Valley (circa 8400D). Carraibhal believes the Valley to be "Her Kingdom", as part of her sick fascination with Thyr. The damage she did was minimal, but her tenacity and cruelty earned her a place in the prison.
==== Classification: Astradaemon (Daemon) ====
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* Name: Krizon "The Kitten"
* Crime: Astradaemons are harvesters of souls, and most prefer to hunt the 'tastiest' souls they can find, often those of powerful incorporal undead. Krizon however valued quantity over quality, and would attempt to devour anything. It's nickname came from the large sack it carried - often full of kittens, chickens or similar - that it would consume before any confrontation. The sheer number of souls made Krizon powerful to the point of being indestructible. Extraplanar aid was sought with which to destroy him; placing it in the prison was one of the stipulations in return for aid in stopping it.
==== Classification: Augmented Humanoid (Planetouched human) ====
* Name: Sword Sister Lady Macry 
* Crime: Attempted genocide/xenocide. Sister Macry aimed to become the vessel for Muir, and in doing so, "rid the Valley of undesirable elements and cement righteous rule by flame and sword"
==== Classification: Ancient Umbral Dragon ====
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* Name: Rannalith the Consumer
* Crime: Rannalith is pretty much the worst thing we've ever come across. It spent centuries with an old copy of a Valley law-book, trying to commit every crime there was. It was largely successful, with the possible exceptions of:
** Entering a boxing tournament with improper gloves
** Zoophilia
** Selling mandrakes in a public space