Anvil lightning is a type of lightning referred to as "the bolt from the blue" because it often appears suddenly from a seemingly cloudless sky.
It is often called a 'bolt from the blue' because it occurs in seemingly cloudless skies — a cloud-to-ground discharge reaching from the top of a thunderstorm and arcing away from the main cloud and striking the ground. It is dangerous because it fools people into thinking the lightning danger is gone because the storm is not overhead.
This type of lightning is a cloud-to-ground discharge. The discharge happens from the top most of the cloud to the ground. Hence the charges that pass to ground are positive. Thus it also known as positive lightning. It occurs during clear skies and it is considered harmful.
There are also rare types of lightning which could spark the extreme forest fires, eruptions of volcano, etc. Scientists have observed a strange and extraordinary type of lightning, named as the Ball lightning. It occurs as a small charged sphere that floats, glows, and bounces off defying laws of gravity or physics.