It's thought that a group of ravens is called 'an unkindness' because back in the 19th century, many people believed that these birds weren't all that caring when it came to their young. From time to time, ravens can expel their young into the world before they're ready to fend for themselves.
Of course, this is actually a complete misunderstood belief from the 19th century born out of a lack of understanding about these birds.
In mythology, ravens were often associated with bad luck and got a reputation for being tricksters - this could also contribute to them being referred to as 'an unkindness'.
It's not only ravens that get the bad reputation as crows in general, when in groups, are referred to as a murder of crows.
Below are the most common names for a group of ravens:
Ravens tend to get associated with death and dark things for a multitude of reasons. The first being their appearance; they are jet black, and their eyes tend to seem like they're always watching and following you. Another reason is that you tend to be around dead things. This is because of their scavenger nature which means they're opportunists for food.
They are also heavily associated with Halloween, and mostly because of their link to all things dark and death.
These birds are pretty smart compared to the average bird because they have a large brain relative to their size. Ravens are known to hide their food, communicate with one another about what's happening around them and have been known to mimic human voices.