Vhagar's Law

"Having grown up surrounded by warfare and death, Vhagar could not envision a better world past it. Therefore, he chose not to try, and entrusted our people's revolution in the hands of future generations. All he did was ensure that no matter their vision, the process would be bloodless."- unknown Moon Reader

Vhagar runs a subtle illusion which creates a compulsion amongst Cotd to never kill one another. The effect is particularly strong within the Darklit Cities, but manages to diffuse across a significant portion of the continent and can functionally be treated as affecting the entire known world. This is a big reason why there are no murders within the Darklit Cities. The land with a population in the tens of thousands averages an annual death toll in the single digits.
The energy used is drawn from 'worship' of him. A lot of higher ups don't really recognize Vhagar to be a god as such, but go through the motions because they see it to be in their best interests.
Corollary to this is the compulsion to back down once backed into a situation in which one really should've died. In other words, it's entirely reasonable to win a duel and hold a blade to the opponent's throat, then sheathe one's sword and walk away without expecting short-term reprisal. Long-term consequences are a different story, though- this feature only exists as a way to de-escalate volatile situations.
The compulsion builds upon existing feelings and cultural beliefs. Even should it be halted, Cotd would still be strongly against killing one another (very firmly in the 'no death penalty under any circumstance' political viewpoint). It relies on the mutilated souls inherent to Cotd (consequently becoming stronger with later generations of the race): while present for ordinary humans, the compulsion is nowhere near as great, and only manifests to a remotely measurable degree within the The Pitch Black Forest proper.
Vhagar's Law is the primary reason groups such as the Infernal Realm (and numerous much smaller cases) can exist. From an outside perspective, they're all bloodthirsty demon-binders, so why is it that they don't simply implode on themselves? As Cotd-descendant, they're literally incapable of plotting to kill each other.

History

Darkness, by nature, consumes its own kind in order to grow. This propensity for cannibalism can be seen in practically every Darkness-aligned organism. One could argue that Vhagar's Law stunted the potential for Cotd growth dramatically- had the race been sharpened by war instead of dulled by peace, it could've ended up with the largest concentration of power in the entire continent- hundreds with the strength of Shrouded, instead of a mere seven. Vhagar's Law is an outright rejection of this idea: a promise that nobody will die for the sake of another's strength. Some believe it to be merely a dead end forestalling the inevitable, pointing to the rise of the Infernal race- while the Cotd reach an asymptotic limit to their population, Infernals can grow indefinitely (so long as they're not the dominant race, limited only by their parasitic nature).
While Vhagar is the only one maintaining the illusion, it was originally kickstarted by several of the Shrouded working in tandem- primarily with the aid of Murmur and Ariadne.
Vhagar's role as its master stems from his widely acknowledged lack of ambition beyond protecting his homeland. The other Shrouded felt that he could be trusted to not overreach, and was overall the most suited to the job. Part of what spurred them to implement it in the first place was not altruism: rather, it was to prevent the Great One controlling Cotd afterlife from obtaining further strength.