Blood sacrifice

It is said that a long time ago The Cult of the Old Gods sacrificed living beings to please the old gods. This was done to keep the old ones appeased during their slumber.
The population of the realm lived in fear of having their children abducted and used in one of these horrific rituals. The ancient texts describe these sacrificial rituals in great detail.
The members taking part in this ritual would paint their faces with ash to be black as night.
Three or more shamans would start chanting one of their sacred songs while one would start slowly carving chunks of flesh from their victim. Ancient eldritch spells would keep the person being sacrificed alive for as long as possible, but doing nothing about the horrifying pain they were in. The screams of torture are said to have pleased the old gods and they would reward the leading shaman with immense powers.
Thankfully, these kinds of rituals are a thing of the past. The Emperor and his knights hunted down as many shamans of The Cult of the Old Gods as they could and forced the rest into hiding. None of these horrifying rituals have been reported in over a thousand years.