Two yellow cards for punching
Underdogs Castres completed a miserable week for Clermont by defeating the favourites 25-9 in Nantes on Saturday to reach the French Top 14 Final against Toulon.
Smarting from their painful 15-16 defeat to Toulon in the European Cup Final a week ago, which denied them a first-ever continental crown, Clermont failed to produce their trademark swashbuckling rugby as they limped out of the competition with barely a whimper.
Clermont had finished top of the table during the regular season, scoring the most points and conceding the fewest.
Having beaten Castres comfortably at home earlier in the campaign, although they were narrowly beaten away, they were expected to storm to victory, despite nursing a hangover from their defeat in Dublin a week ago.
But South African scrumhalf Rory Kockott kicked 20 points and centre Romain Cabannes raced 70 yards to touch down the only try of the game 16 minutes from time after intercepting Mike Delany, who kicked three penalties for Clermont.
Clermont were a mere shadow of their normal selves and suffered a nightmare afternoon, particularly in the scrum where Castres dominated.
It means coaching pair Laurent Labit and Laurent Travers may yet bow out in ultimate style before leaving to take over at big-spending Racing-Metro next season.
Having reached the quarter-finals in 2010 and 2011, and then the semi-finals last season, Castres are now in an unexpected final, although they will have their work cut out to deny a Toulon side riding the crest of a wave following a first-ever European title last week and a 24-9 success over reigning champions Toulouse in their semi-final on Friday.
A measure of Clermont's troubles was evident from the first quarter-of-an-hour as scrum-half Morgan Parra, usually so reliable with the boot, missed two penalties.
That allowed Castres to take a 6-0 lead as Kockott proved unerring himself from the tee and he had notched four successful kicks by half-time as Delany took over from Parra and kept Clermont in touch, although they trailed 12-3 at the interval.
Any hopes a second Delany penalty on 46 minutes would change the flow had already been extinguished by a fifth kick from Kockott, who added a sixth before Delany's third on 59 minutes.
Trailing by nine points, Clermont really needed a try to turn the momentum in their favour but Delany's overly ambitious pass to Wesley Fofana was cut out by Cabannes and with that Clermont's hopes went up in smoke.
Castres have not won the French title for 20 years and are looking for a fourth crown. Toulon likewise are going in search of a fourth title and their last win was the year before Castres' in 1992.
Pens: Delany 3
Pens: Kockott 6
Yellow cards: Julien Bardy (Clermont, 77 - foul play, punching), Antonie Claassen (Castres, 77 - foul play, punching)
Clermont: 15 Lee Byrne, 14 Sitiveni Sivivatu, 13 Aurélien Rougerie (captain), 12 Wesley Fofana, 11 Naipolioni Nalaga, 10 Mike Delany, 9 Morgan Parra, 8 Damien Chouly, 7 Julien Bardy, 6 Julien Bonnaire, 5 Loïc Jacquet, 4 Julien Pierre, 3 Davit Zirakashvili, 2 Benjamin Kayser, 1 Raphael Chaume.
Replacements: 16 Ti'i Paulo, 17 Thomas Domingo, 18 Nathan Hines, 19 Alexandre Lapandry, 20 Ludovic Radosavljevic, 21 Brock James, 22 Benson Stanley, 23 Daniel Kotze.
Castres: 15 Brice Dulin, 14 Romain Martial, 13 Romain Cabannes, 12 Seremaia Baï, 11 Marc Andreu, 10 Remi Talès, 9 Rory Kockott, 8 Antonie Claassen, 7 Yannick Caballero, 6 Ibrahim Diarra, 5 Rodrigo Capo Ortega, 4 Christophe Samson, 3 Karena Wihongi, 2 Brice Mach, 1 Saimone Taumoepeau.
Replacements: 16 Mathieu Bonello, 17 Yannick Forestier, 18 Joe Tekori, 19 Jan'nie Bornman, 20 Romain Teulet, 21 Daniel Kirkpatrick, 22 Paul Bonnefond, 23 Mihaïta Lazar.
Referee: Christophe Berdos
Assistant referees: Patrick Pechambert, Sébastien Minery
TMO: Eric Gauzins Clermont