A long time ago, before the land had been irreversibly damaged, there was a great catastrophe that occurred called the Mushroom War. The war resulted in the near annihilation of the entire human species and irreversibly changed the world into the land of Ooo once the final weapon of the war, the mushroom bomb, was detonated. In the presence of the radiation, a great evil emerged. He was referred to as the Lich King. Something similar has happened to you, perhaps from exposure to the radiation of the mushroom bomb or some other method, you have become an existence similar to that of the Lich. A herald of death, destruction, and the end of all, an undying physical manifestation of decay.
Your body has changed into that of decay and death, giving you access to strength beyond that of a mortal. As you have just started your adventure into being undead, you are only capable of destroying islands with your physical might. However, through time and growing your necrotic energy, you can do so much more than break islands. You have enough speed that you could easily move at speeds that others with enhanced senses beyond those of humans would only be able to assume you teleported. Your durability is also nothing to sneeze at, as you can take hits that would break continents with just a scratch and regenerate in seconds. Like all undead, you possess unlimited stamina and have no need to breathe, eat, or drink to survive.
Through your necrotic energy, you possess a variety of abilities, such as being able to manipulate fire, radiation, the minds of others, necromancy, induce paralysis in others, nullify the powers of others, create illusions, stop and manipulate time and darkness, manipulate objects with your mind, and so much more. As you grow more experienced in necrotic energy, you can do practically anything that the lich is seen doing or even do things he hasn't seen by exploring the magic that you now possess.