Evil is one of the two cosmic factions, the other being Good.
The central tenet of Evil is that those with Power should wield it as they see fit. While this most commonly leads to corruption, self-service at the expense of others, and megalomania, Evil is equally supporting of any agenda so long as the one holding it is true to themselves and proves themselves stronger than any who oppose them. Evil upholds service to the self above all else. The Gods Below are proponents of individual strength and the ability to claim a future for oneself, morality does not enter the equation.
At its most extreme Evil holds that the only right in the world is that which cannot be defeated; the only sin is defeat, and the only virtue is victory. All else is permissable.
Evil in creation is not necessarily that of earth, it does not require cruelty or monstrous acts. But those beings who most exemplify it have no qualms to utilize monstrous acts to get what they desire.
Evil tends to act in defiance of social and normative pressure. While Above will champion the cause of heroes directly, the Gods Below will provide power to any who serves their purpose and allow a victor to emerge between them. While Good prevails against the odds, Evil creates and takes advantage of situations where it cannot lose or benefits no matter the outcome.
The worship of Evil and the Gods Below is more personal than Good and it's House of Light. There is (in general) no such thing as a grand temple of evil or a priest of evil, as everyone is allowed to choose whether to worship the Gods Below and how devotedly, without anyone else knowing or judging. The Drow are a notable exception with a very defined religious structure in their sister goddesses Sve Noc and the Tenets of Night.
While Good has the choirs of angels who can be petitioned or who will offer power directly when the odds are flipped against heroes. Evil can summon Devils and Demons. These beings are not benevolent and will destroy their petitioner unless bound and controlled, and in the case of Demons, are directly damaging to the fabric of Creation. While generally only priests or heroes may petition Angels for miracles, any with appropriate knowledge may summon forces from the Hells.
Evil Named do not die of old age or natural causes. Instead, they live until they lose their Name or someone kills them. While this means that villains have theoretically unlimited lifespans, they rarely live much longer than heroes because both heroes and competing villains will try to kill them.
Evil does not gain the same level of healing power as Good. Instead of healing and resurrection, Evil gains necromancy and similar abilities to partially defeat death. Deals with devils can also grant incredible power in many disciplines of magic, but almost always at a cost. Most commonly this cost is paid in a great physical toll associated with calling on pacts, but it is also possible for a pact made foolishly to drive a villainous mage completely mad, as was the case with Dread Emperor Tenebrous.
Most importantly, the Roles of villains tend to be tied to achieving impressive things before eventually being defeated. As such, they have an easier time amassing power and using it than heroes do, but heroes are better at thwarting their schemes in the final hour and undoing their greatest works.
The Night is a power drawn from the Gods Below, and is in many ways a power that is aligned in opposition to the Light of the Gods Above.
The most important beings aligned with the power of evil are demons and devils. In each of the 24 Hells, there are a vast, possibly infinite number of demons of a single type, and there are a vast number of lesser Hells populated by infinite numbers of devils.
Devils are comparatively minor threats, While their diversity and strangeness make them difficult for conventionally trained soldiers to deal with, a force experienced in dealing with devils can face considerable numbers of them with acceptable casualties.
Demons are immensely more powerful and dangerous. Each variety has different unique powers, all variations on a theme, that can generally affect vast areas in a reasonable amount of time. Most importantly, Demons are directly harmful to the fabric of creation itself, and contaminate regions and the people therein with their mere presence, all of which requires quarantine and sanitization. Demons are strong and resilient, requiring teams of Named to defeat even a single one, if they can even best it while avoiding its power. Even if a demon is defeated, it requires a fairly difficult use of magic to seal one, and only Heroes may permanently kill a Demon.
Evil has no analogue to the power of angels.
Many nations are generally considered to be devoted to Evil. These include: