A serious-looking injury to Gregor Townsend compounded the Border Reivers' misery as they lost to the Scarlets.
Reivers: (8) 15:
Tries: Brown, MacDougall
Scarlets: (14) 24:
Tries: J. Davies, Havili
Pens: Hercus 2, B. Davies
The home side led 8-3 thanks to a Charlie Hore penalty and Kelly Brown's try when Townsend was forced off.
But John Davies' try was converted by Mike Hercus, to add to his earlier penalty and he and Barry Davies added further points with the boot.
Aisea Havili went over for Scarlets, with Hercus converting, rendering Ben MacDougall's late try meaningless.
Townsend, playing his first competitive home game since returning from France, lasted only 10 minutes, having been trapped in a pile of bodies when he was hauled to the turf just inches short of the line at the end of a thrilling 30-metre surge.
Brown was perfectly positioned to claim the touchdown when the ball was slickly recycled but there was more concern about Townsend's condition.
Reivers had made a bright start with Hore confidently slotting a penalty for diving in illegally at a ruck.
The lead lasted only three minutes, however, as USA marksman Hercus slotted the equaliser.
Despite having Dave Duley yellow carded for a rucking infringement, the Scarlets' John Davies was driven over by his pack-mates and though Hercus was wide with the conversion, he made ample amends in the 38th minute by banging over his second penalty.
Worse was to come for the Scots when long-range specialist Barry Davies powered over another accurate kick from almost 60 metres.
The Scarlets began to up the tempo and they effectively claimed the glory when Hercus chipped over an opportunist drop goal.
In a frantic finale, Havili barged over for another Scarlets try, goaled by Hercus before Hore added the extra points to a consolation effort for the Reivers by MacDougall.
Border Reivers: S Moffat, S Danielli, B MacDougall, C Hore, G Law; G Townsend,
C Cusiter; P Thomson (capt), R Ford, B Douglas, O Palepoi, C Stewart, S Gray,
W McEntee, K Brown.
Replacements: Replacements: C MacRae for G Townsend (10), T McGee for P Thomson (69),
S Scott for R Ford (74), A Rennick for O Palepoi (74),
Dalziel for Stewart (66), A Miller for W McEntee (52).
Not Used: B McKearchar.
Scarlets: B Davies, T Selley, D James, R King, A Havili;
M Hercus, C Stuart-Smith; I Thomas, C Hawkins, J Davies, I Afeaki (capt),
A Jones, J Mills, G Thomas, A Popham.
Replacements: M Watkins for T Selley (52),
C McDonald for C Stuart-Smith (80), D Duley for A Jones (15),
G Quinnell for J Mills (67).
Not Used: Williams, A Yelland, G Bowen.
Referee: S McDowell (IRFU).