Barry Davies scored a stunning try as the Scarlets clinched the final place in the Heineken Cup quarter-finals at the expense of Northampton.
HEINEKEN CUP, POOL FOUR
Pens: Grayson (3)
Tries: D Jones, B Davis
Cons: S Jones
Pens: S Jones (2)
Davies bravely slid in on the halfway line to claim a chip by Saints' Paul Grayson before regaining his feet and scorching over under the posts.
Dafydd Jones also crossed for the Welsh side as they out-scored their hosts by two tries to nil.
The Scarlets will now host Biarritz at Stradey Park in the last eight.
After the snow and rain of the previous week the pitch was always going to be heavy and it was a very physical affair, with the two packs hammering away at each other for the full 80 minutes.
Grayson missed a penalty after just three minutes but popped over a simple effort a minute later to get the Saints on the board.
The Scarlets did not touch the ball for the first six minutes but once they secured some possession they put Northampton under sustained pressure and scored their first try of the game.
The visitors moved the ball first one way and then the other, with Dafydd Jones striding over, despite the attentions of Steve Thompson, to round off a fine move.
Stephen Jones was unlucky to hit the post with his conversion attempt but he soon slotted a long-range penalty to nudge the Scarlets further ahead.
Grayson had had a rocky time with his kicking and he blotted his copy book for the second time when he pushed a simple penalty wide after half-an-hour, but he slotted two penalties either side of half-time to edge Northampton ahead 9-8.
With rain falling constantly the game became even tighter and it took a moment of genius from Davies to rip apart the Saints' defence.
Stephen Jones had nudged the Scarlets back into the lead before Davies fielded Grayson's chip and raced over for the game-breaking try.
Stephen Jones' conversion gave the Scarlets an 18-9 lead and as Northampton tried to get back into the game the former champions made a series of errors.
Bruce Reihana nearly scored a late consolation try for the hosts but the Scarlets dominated the last half hour as they ensured a Welsh presence in the Heineken Cup quarter-finals.
Northampton: Reihana, Tucker, Hyndman, Leslie, Cohen, Grayson, Robinson, Smith, Thompson, Morris, Lord, Connors, Blowers, Fox, Seely.
Replacements: Richmond, Budgen, Phillips, Soden, Dawson, Drahm, Ripol.
Llanelli Scarlets: B. Davies; Evans, Taylor, Watkins, Finau; S. Jones, Peel; I. Thomas, McBryde, J. Davies, Cooper, A. Jones, D. Jones, Easterby, S. Quinnell.
Replacements: John, A. Gravelle, Wyatt, Hodges, Phillips, G. Bowen, Selley.
Referee: Donal Courtney (Ireland).