Watford booked their place in the Championship play-off final after an incredible conclusion to their 3-1 win over Leicester in Sunday's semi-final second leg at Vicarage Road.
Gianfranco Zola's team, who were beaten 1-0 in the first leg on Thursday, had levelled the tie at 2-2 on aggregate thanks to two superb strikes from Matej Vydra either side of David Nugent's goal for Leicester.
But the Hornets looked set to suffer a heart-breaking defeat in the sixth minute of stoppage-time when Leicester's Anthony Knockaert stepped up to take the penalty he had won moments earlier.
However, former Arsenal goalkeeper Manuel Almunia saved Knockaert's spot-kick and then blocked the rebound.
Watford responded to that escape by launching an immediate counter-attack which ended with Deeney lashed in the goal which sealed a 3-2 aggregate triumph and sent the hosts through to the play-off final on May 27 against Brighton or Crystal Palace, who meet in their semi-final second leg on Monday.
Vydra had not scored in his last 12 games but the Czech forward ended his drought in sensational style in the 15th minute.
Marco Cassetti chipped the ball over the top and Vydra, on the run, let it drop over his shoulder before crashing a superb angled volley across Kasper Schmeichel and into the net.
That levelled the tie on aggregate but Leicester got their noses back in front just four minutes later, Nugent out-jumping Lloyd Doyley at the far post to nod in Matty James' corner.
Watford were back in front in the 65th minute when Vydra played a neat one-two with Deeney just inside the area and side-footed past Schmeichel.
With extra time looming Knockaert tumbled under minimal contact from Cassetti and referee Michael Oliver pointed to the spot.
Almunia saved Watford's season as he kept out Knockaert's tame penalty with his legs.
Watford charged back upfield and Jonathan Hogg's header back across goal was touched on by Vydra and Deeney slammed in the winner.