Step 1: State his role directly:
Megasthenes was the Greek ambassador Seleucus I Nicator sent to Chandragupta Maurya's court after their 303 BCE treaty.
Step 2: State his arrival date:
He reached India and Pataliputra around 302 BCE, and his resulting work 'Indica' is a key foreign source on Mauryan India.
Final Answer:
\[ \boxed{\text{Greek ambassador at the Mauryan court, arrived c. 302 BCE}} \]