Which sentence makes sense, the first or the second?
- I am legend.
- I am a legend.
Either works, but they have different meanings, and I am a legend is probably the one you want.
Legend (Dictionary.com) means:
1. a nonhistorical or unverifiable story handed down by tradition from earlier times and popularly accepted as historical.
6. a collection of stories about an admirable person.
7. a person who is the center of such stories
“She became a legend in her own lifetime.”
I am a legend would use definition 7 above, meaning that I am an amazing person.
I am legend would use definition 1 or 6 above, and would mean that I embody unverifiable history or myth, that my whole existence is shrouded in fable. This isn’t a normal thing to say. While this concept is too stilted or dramatic for usual speech, there is a 1954 novel and a 2007 movie called I Am Legend, along these very lines.
It’s confusing whether you meant to ask about a vs. an, since the question doesn’t include this debate, but you should never say I am an legend, as a is always used before a consonantal sound.