Merwyn the Malicious is a an outspoken villain from the Harry Potter series. He is a Dark Wizard that is said to have performed unpleasant spells and jinxes in his time.
Little is known about Merwyn other than the fact that he is a dark wizard. He probably may have tortured or even killed other witches and wizards with his dark magic. A portrait of Merwyn is found at Hogwarts and he is also shown on a Chocolate Frog Famous Wizard card.
It is unknown when or how he died but since his portrait is seen at Hogwarts it can be assumed that he was an important wizard in the wizarding world.
Some Harry Potter fans think he is related to Salazar Slytherin, but this is not proven or revealed by J.K Rowling.