Spell out numbers instead of using numerals (because this is fiction, not journalism, so the rules of the Associated Press Stylebook don’t apply here): four hundred, not 400.
Hyphenate all of four-hundred-year-old (compound adjective).
Change the comma after line to a period and capitalize She after it.
She is a four-hundred-year-old demon born to a dying line. She disguises herself as an ordinary citizen, living in the suburbs and tending to a huge collection of pot plants.
Just FYI: Once or twice, I’ve seen someone confuse pot plant with potted plant. There’s a difference! I left the original spelling intact because it amuses me that passing as an ordinary citizen would include growing a lot of a certain herb… 🙂