Thunderbolt Fans is a boss from Darius Force. It is formed by two sea anemone-shaped units, one big and one small, standing over a floating platform.
The first target to destroy on the boss is the smaller, purple unit, though it stays out of reach for most of the time. At first the boss will move around and shoot a few bullets in a small spread. After a while the smaller unit will detach from the bigger one and float to the left, leaving to floating mines between them, after which the bigger unit fires a lightning bolt which goes through the mines and into the small unit. Once the attack is over the small unit reattaches to the main body and the pattern repeats.
With the small unit destroyed, the larger unit becomes vulnerable. It will continue with the bullet spread but will also shoot the lightning bolts randomly into the screen until it is defeated.