Morgan Stoddart's first-half red card saw a spirited Scarlets' challenge end in a comprehensive 49-16 defeat at Clermont Auvergne.
The Welsh side were more than competitive for the first 37 minutes at the Stade Marcel Michelin, but Stoddart's dismissal for a second yellow effectively ended their hopes of an heroic Heineken Cup upset.
The scores were level at 13-13 when the Wales wing - who was making just his second appearance since a 15-month injury lay off - saw red, but last season's beaten semi-finalists ran away with the game after that to secure a comprehensive bonus-point success.
Jon Davies' early try was the highlight for the Scarlets but Clermont crossed for six scores of their own, with Damien Chouly, Lee Byrne, Julien Bonnaire, Napolioni Nalaga and a penalty try adding to Sitiveni Sivivatu's first-half score.
The Scarlets enjoyed the perfect start as Davies crossed for the game's opening try with just five minutes played. The much-vaunted Scarlets back line did the damage, with strong handling from Wales stars George North, Scott Williams and Priestland providing the platform for Davies to step inside two tackles on his way to the line.
Stoddart received his first yellow card for a cynical offside on the quarter-hour mark and Parra made him pay for his indiscretion by slotting the straightforward penalty. The visitors survived Stoddart's first 10 minutes in the sin bin and led by an even greater margin by the time he returned as Priestland slotted a 20th-minute penalty and a 22nd-minute drop goal.
Sivivatu's converted try levelled things up as the former New Zealand wing combined with Parra after the scrum-half took a clever inside ball from a lineout close to halfway. But the match turned before the break as Stoddart saw yellow for failing to roll away after making a tackle yards from his own line and referee Peter Fitzgibbon followed the yellow with a red.
Parra's penalty handed Clermont a 16-13 lead at the interval and they increased that advantage just after the sides returned courtesy of a penalty try. Nathan Hines' charge-down of Priestland's clearance provided the initial platform and the pack did the rest by forcing the Scarlets' front row to buckle under pressure.
Clermont effectively put the game to bed after Priestland had kicked another penalty as Lee Byrne touched down against his old club on 48 minutes. Again, Sivivatu provided the telling contribution as his wrap-around pass saw the Wales and Lions full-back race down the right touchline.
Clermont were celebrating the bonus-point with 25 minutes still left to play when number eight Damien Chouly powered over from close range. Although the Scarlets refused to lie down further scores from Bonnaire and Nalaga added an even greater gloss to Clermont's winning margin.