What is a group of ravens called?

A group of ravens is most commonly called an unkindness or a conspiracy.

• Unkindness: This term is thought to originate from folklore and perceived negative characteristics associated with ravens, such as their scavenging behavior and association with death. More information about Unkindness
• Conspiracy: This name alludes to the raven's intelligence and seemingly scheming nature, as they are known for their problem-solving abilities and complex social interactions. Further details about Conspiracy

While these are the most common terms, you might also occasionally see them referred to as a treachery or a murder (though murder is more commonly used for crows).