In the beginning, the world was made of beauty and of magick. Before there were humans, there were gods and those who served them. At war with each other, they fought until a new breed was born out of their senseless violence. Chthonians by name, these new creatures came from the earth that flowed red with the blood of the gods.
The Chthonians rose up and divided the world between the gods—they split the world between themselves.
To maintain the peace, the soldiers of the gods were ordered to be put down. None were to survive. Chthonian law took precedent and together, they were able to bring peace to the world once more and to protect the new life form of mankind.
But the Chthonians were not without corruption. Nor were they infallible.
It wasn't long before they bickered too.
Created by the original gods of light, a Chthonian's purpose is to keep everything—including the gods—in check. Whenever a god has made an action that can hurt or potentially kill another god, upsetting the balance of the universe, a Chthonian is given the charge of exterminating said god. They are generally not interested in human affairs. Chthonians are by no means controlled by a god. You get their attention if you're the primary Source, or if you can pay a high enough price. Chthonians are part of a deity that is in between. They generally keep to themselves, and aren't to be trifled with.
Around the fall of the Sumerian pantheon, the Chthonians were engaged in civil war, which resulted in only eight of them left (including Acheron). The creation of the Machae (War/Polemus, Ker, and Mache) also removed large portions of their population. The count has increased to nine from Styxx dying and being reborn into his true powers. To kill one of them you'd have to be a servant of the Source or another Chthonian.
The Chthonians are born mortals or humans, and are never born of a god (with the exception of Acheron, who absorbed Styxx's god-killing powers while in the womb of the Greek Queen Aara).
Chthonians are considered mortal gods who are bestowed with the power to kill gods and sending their power back to the primal Source without disrupting the balance of the universe.
Chthonians respond and draw their power from the primary Source. It has been shown by Savitar and Takeshi that their powers rival or even surpass gods. While an average Chthonian's powers are unknown, their unrestricted powers are shown to easily incapacitate even a small group of gods.
Acheron and Savitar can see the future, but this is not confirmed as a standard Chthonian power or unique to Savitar and Acheron, as the former is one of the first Chthonians and whose origin is unknown, while the latter is the Final Fate.
They can only be killed by a servant of the primal Source or another Chthonian. War was able to kill them, even in large groups, for he was a servant of the Source. Their powers also do not work against the Malachai, as they are creatures of pure darkness and the ultimate weapon of the dark gods.
The Chthonians also depend on the existing gods to maintain full power. It is revealed by Apollo in One Silent Night and again in Styxx that killing a god weakens the Chthonians' collective power, so they try to avoid killing one as much as possible.