As a power, darkness is typically seen as the “shady” or evil power. The antagonist or assassins use it, but limiting the ability to one side of a spectrum is a disservice.
Look at Raven from team titan. Her powers were considered dark, but she didn’t let that define her. Good writing and world-building will make the darkness more than the opposite of light and be used for evil. In the system, while darkness magic could be used for destruction and curses, it was also vital for a healer to have control of darkness to eliminate disease, to remove dead tissue.
Due to the nature of the dark, users of darkness may be more assertive at night and weaker during the day, or even useless.
Some of the powers used can range from solidifying shadows to moving between shadows and using them as a cloak. Shadow-type abilities typically are not built to destroy large areas or even do a lot of damage at once, while they tend to specialize in is hit and run tactics or slowly weaken the enemy until they can not fight. They are known for damage over time type skills, controlling the battlefield. They can also be known for burst damage, so they make one significant attack, but if that doesn’t work, they are out for the rest of the fight. They could be known for drawn-out battles. Darkness is complex, and as soon as you underestimate them is the moment that you will see the end.
Depending on the world, dark powers can connect to a different plane of existence where shadows are the only thing that exists. This can be used for holding and summoning undead to a place where they can use to teleport in the main world, this plane can serve as a hub for travel since shadows are everywhere.