To sever something means to cut it off completely.
Sever has a strong and serious feeling. It usually means something bad has happened.
Examples:
He will sever the rope with a knife.
The storm will sever the power lines.
She had to sever ties with her old friends.
I accidentally severed the cable with scissors!
The dragon's fire breath can sever anything in its path.
The knight will sever the dragon's head with his sword.