But Ivan Toney, whose 31 league goals this season set a Championship season record, calmly netted from the penalty spot 10 minutes later to begin Brentford’s comeback.
Bournemouth went to 10 men just before the half-hour mark when defender Chris Mepham was given a straight red card for taking down Bryan Mbeumo, who was headed in alone on goal after dispossessing Mepham.
With the man advantage, the Bees controlled play and it seemed a matter of time until the goals came. Vitaly Janelt scored five minutes into the second half to even the aggregate score, and Marcus Forss scored the winner in the 81st.
Brentford has not played in the top tier since 1947 and returns to Wembley less than a year after losing the playoff final to Fulham.
Swansea held on for a winner-takes-all shot at Premier League football on May 29 by drawing 1-1 at the Liberty Stadium for a 2-1 aggregate victory.
Matt Grimes had doubled its first-leg lead with a superb strike before halftime, but Barnsley was given hope when Cauley Woodrow fired home 19 minutes from the end of a full-blooded affair.
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