Setting: frontier planet Evergreen. It's a Gaia world with rich volcanic soil, long growing seasons, lush jungles with many strange and dangerous animals up to and including dinosaurs.

The law enforcement on Evergreen are called Colonial Rangers. Many have territories many hundreds of kilometers across.

Scores of colonists start to go missing near the vast swamps at the mouth of the long, slow Toulon River that crosses much of the continent. This is of special concern because the city of Sialis, 3rd largest city on the planet is built at the mouth of the river and is the planets most important port.

The local police begin investigating and twelve of them go missing in a few days.

The Colonial Governor wants answers and orders the Rangers to make a special investigation.

A team of twenty Rangers arrives to find that much of the cities outskirts have been abandoned. A siege mentality has set in among the people who lock themselves into fortified buildings at night and don't come out until well after dawn.

Almost immediately two Rangers go missing and more than one hundred more are called in from all over the planet.

Rangers aren't stupid. They are well educated in forensics and the ecology of Evergreen. They think they are looking for a predator but it's something new. In the past they had known another kind of Evergreen predator to use limited psychic powers to fool their prey.

There is another disappearance and the rangers arrive while the crime scene is fresh. They find evidence in the swamps that large snakes had been there very recently- and something else: human tracks.

The Rangers were issued helmets that block psychic signals and began to aggressively patrol and send up drones. The abductions stop and several very disturbing sightings are made.

Huge, never before seen snakes were discovered. Easily as large (or larger) than old earth anacondas, some of the snakes were fifteen meters long. The humans with them were being controlled and used as lures.

After some time spent using reconnaissance drones and following the creatures, their nest was found. Rangers were called in from all over the planet to wipe out this menace.

They attacked a grotto where thousands of the psionic snakes and their human slaves were. It was a huge mess and many Rangers were killed but the nest was wiped out except for juveniles captured for study.

The humans who were not eaten, were being controlled by a combination of the snakes psionic powers and a venom that makes them susceptible to it. The longer they were on the venom, the more addicted to it they were. The ones who had been on the venom the longest died of systemic shock. The others faced a long, unpleasant withdrawal process.

They also felt a great deal of guilt for have collaborated with the snakes.

It would be a long recovery for the victims and another hazard that the colonist on Evergreen would have to look out for. Over three hundred colonist and thirty-two rangers lost their lives but, compared to other threats, they were lucky.