Four Lords of the Diamond

In the future suggested in these books, mankind has gone out to settle planets across a large part of the galaxy, terraforming them when possible, and the resulting empire, known as the Confederation, has established a civilization where everything and everybody is as constant and uniform as possible. (However, they realized that this wasn't possible everywhere, and they maintained a number of non-standard worlds on the fringes of their territory.)

The Warden system is one place where this didn't work. Its discoverer, a deep space scout named Warden, was known for his quirky, terse reports, but the Confederation tolerated this, because it provided a role for people like that, and it was hard to find people willing to live their lives really, truly isolated on these single-man scout ships.

His preliminary report of this system was a symbolic code, "4AW". This meant that there were four human habitable planets in the system, an unheard-of event.

Eventually he gave his full report, including the names he gave the planets:

"Charon. Looks like Hell.
Lilith. Anything that pretty's got to have a snake in it.
Cerberus. Looks like a real dog.
Medusa. Anybody who lives here would have to have rocks in his head."

At the end, he gave another special code, "ZZ", which indicated something very odd about the place, but not anything specific.

When the first settlers arrived, they found all four worlds oriented 90 degrees apart around their sun, and thus the worlds became known as the Warden Diamond. (The formation was coincidental and not repeated again within the length of the story, but the name stuck.)

Later, when the system was settled, they discovered what was so strange about it. Lilith was the home of a strange microorganism which inhabited literally all matter on the planet, and maintained it in a pristine state. The settlers were seen as alien to the system, and as living matter, they were kept alive by the Warden organism, but all of their equipment becan to decay and fall apart once the organism had had enough time to adapt to decomposing such forms of matter.

The organism was unknowingly spread to the other planets in the system by the original settlers, where it was unable to transform those worlds into mirrors of Lilith, so they adapted to maintaining the other worlds as they were.

The first attempt of somebody to leave the system was a horrid failure, as the Warden organisms died when they traveled farther than a certain amount from that star, and this caused the horrible, grisly deaths of all the travelers on that ship, as their cells had been adapted by the Warden organism for its own use, and could not survive without it.

The Confederation dealt with this by turning this system into a penal colony, sending all the misfits from the uniform worlds, as well as criminal geniuses from the fringe worlds who were deemed too criminal and captured.

This all went fine, until the Confederation found evidence that they were being infiltrated by an alien race which had dealings with the Warden worlds. They sent in a top spy to investigate these worlds and to attempt to assassinate their leaders, called lords, or thus, the Four Lords of the Diamond.