Here's a breakdown of the sentence:
* 僕 (boku): This is the first-person pronoun, meaning "I". It's a more casual form of "watashi", also meaning "I".
* は (wa): This is the topic marker, indicating that "boku" is the topic of the sentence.
* 鬼 (oni): This means "demon".
* です (des): This is the copula, indicating that the sentence is making a statement of fact.
Important Note: While "oni" is often translated as "demon", it can also be translated as "ogre" or "monster". The specific nuance will depend on the context.
The sentence is a very strong statement, likely to be used in a situation where the speaker is trying to make a point or express their identity.