Lilith is a villain that has popped up in many different pieces of pop culture, the most notable being True Blood and Supernatural. In the former, she's considered the first vampire, a very religious figure for the creatures of the night. In the latter, she's a very powerful, evil demon.
Historically, Lilith has been many things. In Christianity, she was thought to be Adam's first wife, before Eve. According to the stories, God banished her from Eden for being too willful. In other myths throughout history, she adheres more to Supernatural's interpretation: one of the oldest, most evil, most powerful demons.
Here's a fun kicker: in some Occult traditions, Lilith has been thought of as the wife of Satan, perhaps even his mother, grandmother, or concubine. This would absolutely be a good call for Mallory, as if she were cut from the same cloth as Michael. But since she might be higher up in the ranks than Michael, the son of Satan, it would explain why she's more powerful.
All things considered, one thing's for sure: Mallory may be the key to unlocking the season of Apocalypse as a whole. We can't wait to see what's revealed in the episodes that lay ahead.