Rovers were 18-10 ahead in the Million Pound Game against Salford Red Devils with just 96 seconds remaining, their safety seemingly secured.
Indeed, their tiring opponents, out-played for most of the second period, looked dead, buried and destined for the Championship.
However, desperate Salford somehow managed to score two tries in those final two minutes via Niall Evalds’ second and Greg Johnson to level.
There seemed to be a forward pass in the latter but referee Phil Bentham award the try with the winger touching down after the hooter had already sounded.
It was left to Gareth O’Brien to try and win it for Salford with his touchline conversion but - to the roar of the Rovers crowd - it fell wide, sending the dramatic contest into golden point extra-time.
However, Hull KR’s fans were silenced when - after Salford received the kick-off - the visitors worked upfield for O’Brien to deliver a remarkable drop-goal from almost 50m out.
Salford’s fans were delirious - unfortunately some spilled onto the pitch and fought with rival fans in ugly scenes - and they were saved.
After all that tension, dread, fear and anxiety, Rovers’ worst fears were realised - they are out of Super League for the first time since 2005.
There was nothing pretty about the game - in every sense of the word - but it was never likely to be given the sheer magnitude of the prize at stake.
Rovers, with an immense showing from hooker Shaun Lunt and closely followed by prop Dane Tilse, rarely looked comfortable and they were scrappy at times but seemed to have done enough.
A Josh Mantellato penalty at the end of the first half, that gave them a 12-10 lead, and Thomas Minns’ 57th minute try - the centre’s 19th this term - gave them some breathing space. They thought.
Webster’s side grew in confidence, Salford erring more and more as the pressure got to them badly.
Minns, forced onto the wing after an injury to Ben Cockayne, produced some telling tackles on Evalds to frustrate them further and Rovers just had to negotiate the last two minutes. But then came that stunning finale.
Earlier, Rovers were 10-0 up after just 13 minutes and looked like the only side who wanted to stay in Super League.
Running hard, tackling with the same ferocity and playing with a general urgency, they were far superior to Salford whose first contact in defence was atrocious at times.
Prop Adam Walker barged over from close range inside seven minutes for the positive start they so desperately craved and Josh Mantellato added a second with a fine fine finish following some slick handling down the short-side.
He converted the former and, with Salford’s star centre Junior Sa’u already off having failed to shake an early rib injury caused by Tilse’s tackle, everything looked in the hosts’ favour.
Shaun Lunt - such an effervescent presence - then maid a 50m raid in the restart set and, if Terry Campese had not been held up after stepping over soon after, it could have been game over.
However, barely 60 seconds later, Salford responded with their first real attack, robust second-row Ben Murdoch-Masila charging over far too easily from 20m out, hitting Michael Dobson’s pass to leave Campese trailing in the 18th minute.
Granted, there was a forward pass from Salford hooker Logan Tomkins a couple of plays earlier but the visitors were glad of anything.
Rovers’ second-row Chris Clarkson was placed on report for an alleged crusher tackle on Josh Jones - the former St Helens centre who began to cause Webster’s side greater problems as the half wore on - and Salford should have scored again but Weller Hauraki bizarrely opted not to dive for Dobson’s grubber.
Mantellato was fortunate, too, when he lost the ball on his own lin, Robert Lui touching down, but replays showed the Salford stand-off had ripped possession from the Rovers winger in a two-man tackle.
Still, Salford did draw level when, after Dobson was rather generously given a penalty for being held back after his own break, Gareth O’Brien’s fine cut-out pass put Sa’u’s replacement Evalds tip-toeing in at the corner untouched.
O’Brien nailed the touchline conversion and Rovers needed Cockayne to save them from further damage when he produced a try-saving tackle on Greg Johnson, the Salford winger piercing the suddenly tiring home side again.
That said, Lunt was furious with Mantellato and Iain Thornley after he darted his way through on the last tackle and had no support from his outside backs, O’Brien denying the former England hooker just short.
When the enterprising Lunt then sent Ken Sio into space, Cockayne was only stopped by Josh Griffin’s desperate tackle.
The winger, who joins Hull FC now, held on too long, though, conceding a penalty that should also have merited a yellow card.
It did not and, instead, Mantellato accepted the two points for a 12-10 interval lead.
Rovers lost Cockayne to injury early in the second period and then needed Matty Marsh, seen as Rovers’ long-term No7, to produce a crucial try-saving tackle on a Hull KR’s scrum-half of yesteryear - Dobson.
Webster’s side did well, though, to drop-outs as Salford applied the pressure.
Thomas Minns almost scythed through from Campese’s long pass after Dobson had initially caught the Rovers centre with a rash high tackle but Minns did score after a neat grubber from Graeme Horne, Mantellato improving.
A brilliant Campese 40/20 should have garnered more but Rovers could not capitalise and, so, with seconds remaining Josh Griffin’s flick pass sent Evalds in for the frantic finish.
Hull KR: Cockayne; Sio, Minns, Thornley, Mantellato; Campese, Marsh; Tilse, Lunt, Walker, Blair, Clarkson, Donaldson. Substitutes: Horne, Peacock, Allgood, Jubb.
Salford Red Devils: O’Brien; Johnson, Jones, Sa’u, J Griffin; Lui, Dobson; Burgess, Tomkins, G Griffin, Murdoch-Masila, Hauraki, Flanagan. Substitutes: Kopczak, Krasniqi, Evalds, Kenny.
Referee: Phil Bentham (Warrington)