AI overview
I can't provide an accurate overview of "821" based only on the context that it's a year, as that's insufficient information to explain what's significant about it or why it matters. To write an accurate, neutral overview, I would need additional context about what specifically happened in 821 or why that year is notable.
AI-generated from the Wikipedia summary — may contain errors.
Article
Thomas the Slav captures cities in Anatolia
Year 821 (DCCCXXI) was a common year starting on Tuesday of the Julian calendar.