European try-machine Dafydd James, Gavin Thomas and Barry Davies got their touchdowns during a dominating performance which increased hopes they can go all the way to May's final in Twickenham.
Before that, there's a semi-final to be contested, at Leicester or Stade Francais later this month.
But they've got nothing to fear after marching through the group stage unbeaten and smashing Munster in the loose, the crack Irish province being forced into a whole catalogue of errors.
Alix Popham was named official man of the match after tackling Ireland's highly-rated No 8 Denis Leamy to a near standstill.
It was inspiring stuff as European superpower Munster, who have been in three finals and six semi-finals in the last seven years, were brought to their knees. Although they produced tries for Ian Dowling and substitute Donnacha Ryan in the final 14 minutes, there was no way back.
Popham and the likes of Matthew Rees, Iestyn Thomas, Deacon Manu, Simon Easterby and Scott Macleod had been too strong for them.
With a super platform in front of him, Wales captain Stephen Jones marshalled his back division with aplomb. The outside-half, under pressure for his international No 10 jersey from young gun James Hook after missing the win over England with a broken bone in a wrist, converted all three tries while Davies landed a monster of a penalty.
A three-minute assault - the groundwork had been laid earlier - at the end of the first half brought a crucial 10 points to fire them into a 17-0 lead at the interval.
Munster did lift their game, but they weren't good enough, the Scarlets never looking in danger of losing a thriller that was near Test match standard. Wales hadn't produced a semi-finalist since Llanelli lost to Leicester in 2002 and this was a welcome boost for Welsh rugby following the battering it took in the Six Nations.
It was also a lift for the regions because it was the first time one of them had made the last four of the Heineken Cup. Llanelli had been packed with fans for most of the day with hordes of Munster and Scarlets supporters attempting to drink the town dry. The ground was buzzing three hours before kick-off and the atmosphere, aided by a choir belting out Sospan Fach and other inspirational tunes, built towards a crescendo as the click ticked towards 7.30pm and the most important match a Welsh region had played since the Scarlets were taken apart by Biarritz in the quarter-finals three years ago.
Tension was in the air with few people making a prediction about the outcome with any real conviction as the capacity 10,800 crowd took its seats and places on the terraces. There was a host of personal duels to be fought: Jones v Ronan O'Gara, Dwayne Peel v Peter Stringer, Easterby v Alan Quinlan, and Popham v Leamy, the list was almost endless. Who would prevail? That was the question on the lips as the teams trotted out. Munster wearing unfamiliar grey jerseys.
They formed a huddle with O'Gara, captain in the absence of injured line-out colossus Paul O'Connell holding court as the Scarlets came out to deafening applause, fireworks being lit and balloons let off. English referee Chris White, who controversially called time on Wales in Italy a few weeks ago in a row over the clock, was booed by home fans.
Dynamite Scarlets centre Regan King soon broke through and they were awarded a penalty after Munster failed to roll away at the tackle. But Jones fired a regulation penalty wide. However, they opened the scoring from the drop-out 22, running the ball back at Munster. Peel put in a clever chip for James. The winger caught the ball, beat Leamy and had the strength to force his way over.
There was just over five minutes gone and the Heineken Cup record try scorer had increased his haul to 28. Jones converted beautifully from near a touchline and they were 7-0 up. Perfect!
O'Gara missed the target at the other end after the Scarlets pulled down Munster's renowned driving maul. The holders went for the corner, a line-out and another test of strength when they were awarded a second penalty but infringed themselves. Frantic it had been during the opening quarter-of-an-hour, but the pace began to slow as both sides looked to settle.
Scarlets were having problems with their line-out, while Munster were getting pinged at the breakdown by White and for encroaching offside.
Wales' in-form scrummager Iestyn Thomas was leading an encouraging assault on the opposition front row each time they packed down, and the Munster error count grew as they were harried out of their stride.
A foray into the Scarlets' half brought a penalty, but O'Gara was surprisingly wide from 38 metres. They began to get some territory, but the home defence was holding firm with James dumping wing John Kelly. Munster's scrummage problems surfaced again, when they were penalised 16 metres in from touch on the halfway line in the 37th minute. Up stepped full-back Davies for a blast, and the ball went over via the crossbar.
Scarlets were 10 points to the good and there was ecstasy among their supporters.
It got better seconds later when dangerous runner Rees was halted just short, but they kept the ball alive for flanker Gavin Thomas to reach for the line. White went upstairs to the video ref and Englishman Graham Hughes awarded it. Jones converted magnificently from a narrow angle and it was a deserved 17-0 at the end of a breathless half which neared an international for intensity and pace.
Thirteen crucial points had come in a shade over three minutes and Munster's hopes of retaining their crown were nearly gone. Mark Jones skinned Kelly on the outside when hostilities resumed and was heading for the try-line when White belatedly blew up for a forward pass. Bread Of Heaven rang around the famous old ground as revellers sensed a happy outcome.
Jones chipped over Christian Cullen, but collided with the full-back, who scored four tries when New Zealand beat Llanelli 81-3 here 10 years ago. It looked like a surefire penalty, but White and touch judge Tony Spreadbury decided Jones had intentionally run into the Kiwi.
Munster were all over the shop with O'Gara failing to find touch with a penalty, and their handling deficient. It wasn't until the 55th minute they reached the home 22, a frantic effort finally netting them a promising position. But a dogged and determined Scarlets' defence repelled their line-out drives. Munster settled for an O'Gara penalty to bring them within two converted tries of drawing level.
An air of nervousness crept in and it increased when replacement lock Inoke Afeaki was sin-binned for deliberate offside within three minutes of taking over from the crocked Vernon Cooper. Munster hit them from a turnover with New Zealander Mafi breaking out of defence before Leamy and Cullen combined to send left wing Ian Dowling diving over in the corner. There was an audible sigh of relief from Scarlet hordes when O'Gara missed the difficult conversion.
Davies eased the nerves by crossing following fine work from King, Jones and MacLeod. It was effectively game over. The Scarlets had done the job Welsh rugby demanded.