Roy Keane is known as one of the hardest men in world football.

Ranking up there with the likes of Vinnie Jones, Eric Cantona and even Duncan Ferguson, it's fair to say you didn't want to cross Keane during his playing days.

Renowned for his on-field feuds with Patrick Vieira, butting heads with Sir Alex Ferguson and even his scrap with Alan Shearer, the former Manchester United Captain certainly didn't mess about when he took to the field.

But even after he hung up his boots, it seems, as Darren Bent once found out, the Irishman was just as frightening as a coach.

After retiring in 2006, Keane had a string of managerial appointments, including a two-year spell as Sunderland manager, before joining up with Paul Lambert as the assistant manager of Aston Villa in 2014.

But he would last just 149 days in the job before leaving the club, with The Daily Mail reporting that his departure was largely due to disputes with some of Villa's key players.

And after listening to the club's former striker, Bent, recount one of Keane's two-hour-long following a 2-1 loss to QPR, there's no wonder why.

"He [Keane] was really, really scary," the former England striker told talkSPORT.

"He would just lose it.

"And I can understand why, because when you've been a top player like Roy Keane was, one of the very best, in his head he can't understand why us players at Aston Villa made certain mistakes – it just didn't compute.

"Like: 'How can you not pass from there to there?' He just couldn't see it."

The 38-year old scored 25 goals in 72 appearances for the Villans but recalled one game in particular where the Assistant Manager really lost it.

Bent said: "I remember this one particular game we played against QPR, we lost 2-1 down there and he had us in the changing room, Paul Lambert just stepped aside and Roy started talking…

"And honestly, I promise you, it was a night game and all the lights in the building were off, all the fans who use to wait outside had gone, because we were in there for two hours – he just went off!

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"He was talking about the Lucozade players drink, players getting too many massages, people doing yoga. Ron Vlaar had an injury, I think it was shin splints, and Roy was like: 'How can a player be out for that long with shin splints?!'

"He was going around the room picking faults.

"No one really wanted to make eye contact with him, because the moment that happened he would go for you as well.

"Everyone just sat there for two hours like…"

Listen to Bent's hilarious story in the video below...

While sometimes a rousing managerial speech could usually spark a team into better form, it appears Keane's rant might have had the opposite effect, instilling fear in his striker, Bent.

"I can't even remember what happened, I'm still traumatised by what happened that night!

"Honestly, he was crazy."