Stephen Dobbie, who helped shoot down the Reds at the same stage 12 months ago while on loan at Blackpool, was the man sacrificed after just four minutes of a goalless first leg at the City Ground last Thursday following Neil Taylor's dismissal, but he scored one and set up another goal on Monday night.
The midfielder proved worth his weight in gold - or at least the estimated £90million windfall for the team promoted to the Premier League on May 30 - in an open and electrifying encounter.
David McGoldrick and Fabio Borini had already hit the crossbar at either end before Dobbie's short corner set up Leon Britton for a stunning 28th-minute opener, the Scot then added a well-worked second shortly after to give battling Forest a mountain to climb.
Lewis McGugan's free-kick smacked the woodwork before former Cardiff striker Robert Earnshaw threw them an 80th-minute lifeline. And after Earnshaw dramatically hit a post in injury time, Darren Pratley sealed victory by finishing from the halfway line after Forest keeper Lee Camp had gone up for a corner.
It is the East Midlands' side's fourth failed play-off attempt in nine seasons and a fourth for manager Billy Davies since 2005 having missed out twice with Preston and now with the Reds.
The Swans, 28 years after their only brief spell in England's top flight, are looking to complete an astonishing six-year rise from the fourth tier and could now face a Wembley showdown with south Wales rivals Cardiff, who face Reading in the other semi-final tomorrow night.
Skipper Garry Monk was the only change to City manager Brendan Rodgers' XI, coming into defence as expected after Taylor's unsuccessful appeal against his first leg red card.
Forest counterpart Davies brought in four new faces - McGoldrick, Marcus Tudgay, Nathan Tyson and fit-again Guy Moussi being the men called upon.
Davies went over to rouse the Forest support 15 minutes prior to kick-off, yet there was an electric atmosphere bellowing from all quarters of the Liberty as the two teams took to the field.
And it was the visitors who should have drawn first blood as early as the fifth minute, the unmarked McGoldrick clipping the top of the bar following a 40-yard run from pacy forward Tyson.
Both sides were using their livewire attacking men to good effect and Angel Rangel was next to waste a clear-cut opening, the Spaniard fluffing his lines from eight yards.
Swans then went even closer as Borini's deflected strike off Wes Morgan smashed against the bar.
Chances were coming at either end and the deadlock in the tie was finally broken in the 28th minute.
Dobbie's short corner found Britton on the edge of the area, and the midfielder twice turned Tudgay inside out before planting an unstoppable shot into the far corner which left Camp stranded.
And within five minutes it was 2-0 as Dobbie turned goalscorer, playing a one-two with Nathan Dyer before nutmegging Luke Chambers and firing low across Camp.
Forest should have got a goal back within five minutes of the restart, Tudgay blazing over after another fine run from Tyson.
They came even closer to finding a route back in when McGugan rifled his 20-yard free-kick against the bar.
It was up to Forest to force the issue as the clash entered the final 25 minutes although City were dangerous on the counter.
The visitors got the goal they craved - and deserved - 10 minutes from time through substitute Earnshaw, who drilled home Radoslaw Majewski's throughball.
The striker then hit a post in injury time before Pratley coolly clinched matters from inside his own half with Camp miles out of his goal.