Tom Bradshaw's clinical double piled the misery on Aston Villa as Barnsley claimed a deserved 3-1 win at Villa Park.
The Tykes clinched just their second victory over Villa in their history as they extended the hosts' winless run to nine games.
Adam Armstrong's penalty put the visitors ahead before Bradshaw struck either side of Jonathan Kodjia's effort just before the break.
Barnsley's only previous triumph over Villa came in 1998, but they merited this victory which kept them within six points of the Sky Bet Championship play-off places.
Defeat left Villa seven points above the relegation zone. Despite spending over £60m this season which was supposed to bring them an instant return to the Premier League, they are now 19 points adrift of the top six.
Barnsley sold Conor Hourihane and James Bree to Villa last month, but rarely looked second best after riding out some early pressure.
Adam Davies saved from Henri Lansbury after Marc Roberts' mistake, but Villa were routinely restricted to efforts from distance.
Davies tipped over Albert Adomah's drive and claimed Lansbury's shot before the visitors took the lead against the run of play after 25 minutes.
There had been little attacking intent from the Tykes before Marley Watkins advanced into the area and was brought down by Jordan Amavi. The penalty looked soft, but Armstrong shrugged off any controversy to beat Sam Johnstone.
It rattled Villa as they struggled to regain momentum and it got worse two minutes before the break when Bradshaw flicked in Andy Yiadom's cross to double Barnsley's lead.
Villa hit back instantly when Kodjia converted Adomah's centre, but Bradshaw restored the visitors' two-goal advantage after the break following some calamitous defending.
The hosts failed to deal with a Barnsley free-kick and Bradshaw drilled in from 12 yards. This time there was no immediate response from Villa as Steve Bruce's side continued their freefall.
Aston Villa boss Steve Bruce:
"The reason we've got one of the biggest wage bills in the Championship is we've got to deal with that mentality and expectation.
"I'm convinced they are made of the right stuff and can get better and it's my responsibility to make them better.
"There's been big change at the club for too long. It's the best and biggest club I've had in 20 years in management and arguably the toughest one to turn around. Given time I know for a fact I'll do it.
"It's difficult but that's when you show your mettle, roll your sleeves up, batten down the hatches and get ready to try and ride the storm - because there will be.
"Every man and his dog will have their say but I'm determined. I know what's required in this division, I've been around it long enough."
Barnsley boss Paul Heckingbottom:
"We use the fact we're a small group of players, staff and have a small budget as part of our power and what we are. This is us, what we're given, and how we're going to excel.
"What can we do better than everyone else? We don't shy away from what we're short of, but what we do is use that to work out to how to bridge the gap.
"We had to work really hard for that. I think we had to show everything. We're an aggressive team, we don't play with any fear."