"Maiden campaign" refers to the first or initial attempt at something, usually in the context of a competitive or challenging activity. It implies that it is the debut or first experience of a person or team in that particular endeavor.
After a slow start at Atleti, Griezmann has become a key figure in Diego Simeone's side and enjoyed a magnificent maiden campaign at the Vicente Calderon. (caughtoffside.com)
Building on a strong maiden campaign, the former Valencia man has already bettered his own personal best with four games of the domestic season still remaining. (caughtoffside.com)
Guess you missed Bentekes maiden campaign before his serious injury? (justarsenal.com)