Crown Leeches are small black slug like creatures. They are aliens and there are billions of them living in a cluster of planets. They all serve one Queen which is a gigantic leech, the size of a skyscraper.
Crown Leeches can crawl inside your ear and take control of your brain, taking over your body. You become a host for it. The Crown Leeches come down onto a planet and start controlling the people with the most power. Then they wage war on the world and take the society apart from the inside. When most people are dead drones come down and take the food and bodies to feed to their Queen. The Crown Leeches do not think of themselves as individuals, but as a collective and are glad to sacrifice themselves for their Queen's welfare.
The planets the Leeches live on are not natural planets, but are made up of metal and stone, materials they take from the planets they destroy to build new ones for themselves. When they leave planets after they have ravaged them, they are barren desolate balls of rock, stripped from all resources.
The Leeches want V.G because he regenerates and is a infinite food source to feed to their Queen who is always hungry and must eat. They have tried to attack Earth to get him, but Deedle have beaten them.
Crown Leeches, when they're not inside your brain, are quite weak and easy to squash. If one organ is damaged, the damage spreads and they die. But the sheer amount of them (Three times as many humans are on Earth) makes them a near unstoppable force.
If tons of Leeches attack you at once they will eat through your flesh and lick your bones clean.