Zalgo is depicted as a jet-skinned knight in silver armor, with a corona of black lightning. This, however, is just his human form, not his true form.
In his true form, he is described as having seven mouths speaking in six foul tongues, the seventh singing the song that will end the world, and four arms. His right arms are holding a Black Hole and a candle that sheds darkness rather than light. His left arms are said to be covered in the blood of Am Dhaegar, which may have been an ancient city, another entity like himself, or anything else, up to and including an entire universe. He is also described as an amorphous, eyeless being.
According to Something Awful's Dave Kelly (aka Shmorky), the creator of Zalgo, Zalgo's abilities include corruption of comics, cartoons and illustrations, causing insanity and destroying the world. However, it is quite possible that he possesses more abilities that are just waiting to be discovered. In 2016, for example, an anonymous person found out that Zalgo might have the ability to make your recommend videos on YouTube about him. Whether it be a game, a song, or a creepypasta reading.