NSU (31-24, 19-11 Southland Conference), who stormed back from a 6-2 deficit after the third inning, fell to the fourth seed in the Southland Conference Tournament.
Northwestern State's side of the bracket features the first seed Sam Houston State, Southeastern Louisiana and eighth-seeded Lamar
The Bearkats and Cardinals will face each other in the game following the Demons and Lions.
Cameron Brewer (3-1), who took over in the eighth inning, hit the first batter he faced in the ninth inning.
After a double play up the middle to erase it, he hit Kennon Menard, gave up a single to Kyle Cedotal and walked pinch-hitter Jesse Burratt to load the bases with two outs.
Jacob Seward drew a four-pitch walk to score the run that cut the lead to 8-7, and Fisher Jameson singled to left field to knock in the runs that tied and won the game.
SLU used a big five-run frame in the third inning to jump up 6-2, but the Demons answered with three runs to make it 6-5 in the top of the fourth inning.
Chase Daughdrill led off the frame with a four-pitch walk, and scored on Nick Heath's triple down the right field line.
C.J. Webster, who was 3-for-4 on the day with three runs batted in, knocked in Heath on a single. Webster scored on a double from Edwin Gomez one batter after Joel Atkinson singled up the middle.
Atkinson and Gomez were both caught in run-downs on a fielder's choice from Bret Underwood, and ended the inning.
Caleb Dugas tied the game in the fifth inning on a home run over the left field walk that hit the net in front of the Southeastern scoreboard.
He also knocked in the go-ahead run in the ninth inning on a sacrifice fly to right field in the Demon frame of the ninth.
The ninth inning started on a bunt-single from Joel Atkinson, and advanced to second when Underwood reached on an error by the first baseman.
After Dugas' sacrifice fly, Cort Brinson and Chase Daughdrill each singled to drive in Underwood.
NSU scored the game's first runs in the second inning on Webster's triple to center field.
He knocked in Brinson and Daughdrill; Brinson reached on a double to right center, and Daughdrill pushed one through the left side.
Southeastern cut the score to 2-1 in the second inning on a home run from Sam Roberson before its five-run inning in the fifth.
Northwestern State started Austin Tanner who worked 2.1 innings and allowed six runs on six hits and a walk.
Brandon Smith relieved him in the third inning and pitch 4.2 scoreless innings. He gave up four hits and three walks.
SLU's starter Kyle Keller lasted 3.1 innings, and gave up four runs on five hits with a walk.
Gage Pickett pitched two-thirds of an inning and allowed two runs on four hits, and Matt Pittari pitched 2.2 innings and gave up two hits.
Dylan Hills (5-3) took the win on 2.1 innings pitched with two unearned runs allowed on three hits.
NSU outhit the Lions 14-13, and were led by five Demons who finished with multi-hit games.
Brinson and Webster had three hits, and Gomez, Daughdrill and Atkinson all had two hits on the day.