Viper trees are pretty straightforward in concept, too. They're trees with snakes instead of branches. Part fiend, part reptile, part plant, all hungry. They've also got a rudimentary intelligence, although they only speak the language of Tanar'ri, and crudely at that, it does make parlay possible.
The best part of the entry stems from that: the "Ecology" section is presented as a summary of the results of a Harmonium poll to determine the dietary preferences of carnivorous plants. By going and asking them.
It's another one of those awesome moments that you can really only see in the Planescape setting.
* The original study was flawed by an insufficient sample size for necessary statistical rigour! The Harmonium is hiring capable and daring adventurers to go ask the questions of as many Viper Trees as they can find. If none of them speak Tanar'ri, they get saddled with a ludicrously vulnerable translator, too.
* An old friend was conscripted into the Blood War, and fell in battle. The PCs want to find his body, possibly for proper burial, possibly for resurrection, but there's of course a problem: there's already a large grove of Viper Trees going in the now blood-rich soil.
* Legends speak of a grove of viper trees that managed to fell a small traveling clutch of Mind-Flayers, and gained increased intelligence from the feast, which passed on to their offspring. A sage studying social structures of polycephalic entities wants the PCs to find this grove (wherever it may lie in the infinite layers of the Abyss), and escort him there. One good question to ask might be, exactly how smart are they?
And that's it for the Planes of Chaos monstrous supplement! Stay tuned! I'm going to try to blow through the Planes of Law one at an increased pace as well before slowing it down for the Planescape Monstrous Compendium II.