In September 1915 British forces unleashed a new deadly weapon for the first time, which had already been used by the Germans on another occasion. What was this weapon?

1914 saw the end of the type of moving warfare begun by the Germans in their invasion of Belgium in August.  By the end of the year trench warfare had begun, with the trench line extending from the Channel coast to the Swiss frontier.  Thus the battles of 1915 on the Western Front were attempts by both sides to break out of this.