Darlan, a leading contender for the Champion Hurdle, suffered a fatal fall at Doncaster on what should have been his final outing before the Cheltenham Festival.
Lining up in the Listed 32Red Hurdle as an even-money favourite, Darlan looked a likely winner as he cruised to the last in the hands of Tony McCoy.
However, the Nicky Henderson-trained runner met the hurdle all wrong and crashed to the ground.
Although McCoy escaped unscathed, he stood himself down for the rest of the day.
Henderson, who was visibly upset by the loss of his up-and-coming star, said: "It's some game, isn't it?
"Why is it always the good ones?"
Owned by JP McManus, Darlan was among the top rank as a novice last term and had been expected to prove a major rival for ante-post favourite Hurricane Fly at Cheltenham next month.
The six-year-old graduated to Grade One success when running away with the Christmas Hurdle at Kempton on St Stephen’s Day.
He is not the first star McManus has lost in tragic circumstances, with last year's Gold Cup winner Synchronised losing his life after a fall when running loose in last year's Grand National.
Darlan's death understandably overshadowed victory for reigning champion hurdler Rock On Ruby.
Countrywide Flame had set the early pace, with Rock On Ruby sat on his heels before making his challenge at the top of the straight under Noel Fehily.
He was slightly in front and under pressure coming to the last when Darlan suffered his fall, leaving the 13-8 shot to come home three lengths clear of Countrywide Flame.
Clerks Choice was last of the three finishers.
Rock On Ruby's trainer Harry Fry admitted the circumstances were far from ideal, but was satisfied with his charge's performance.
He said: "In terms of the race, we didn't want a scenario like that and commiserations to all of Nicky's team and AP (McCoy) and JP (McManus).
"As far as Ruby is concerned, we've come and he's done the job.
"He's had a good race and that will bring him on again for the day.
"He's jumped well enough, not as sharp as he can be, but there's improvement there.
"We can take him home and put the finishing touches on him.
"He's a really tough horse. He joined Countrywide Flame turning into the straight and it's a long way home, but he'll keep galloping and keep finding all day for us.
"The whole season has been built around Cheltenham and hopefully we can go there full of confidence."
Fehily said: "It's obviously disappointing to see the other horse fall like that, but we've been in front plenty long enough and he's idling up the straight.
"He's where we want him going to Cheltenham now. His best form is around Cheltenham and he goes there now with confidence up.
"If he's good enough on the day, he'll be bang there again."
John Quinn, trainer of Countrywide Flame, believes his horse must have each-way claims at Cheltenham after attempting to make all of the running.
He said: "I'm very pleased with him.
"It's very hard to make the running when there's two top-class horses on your tail.
"At the last he was five lengths down yet at the line he'd reduced that to three.
"They'd gone a good gallop the whole way and then they were racing from a long way out into a strong headwind.
"We know he likes Cheltenham and he jumped so much better because the ground was decent.
"I think he's heading there with a sporting each-way chance."
The afternoon at least started on a better note for Henderson as he teamed up with Barry Geraghty to win the opening two races.
Utopie Des Bordes, a Grade One-winning chaser in France, landed the 32Red Casino Mares' Novices' Hurdle, while Minella Forfitness stayed on strongly to claim the 32 Bonus At 32Red.com Maiden Hurdle.
Henderson is keen to keep the pint-sized Utopie Des Bordes (4-11 favourite) against her own sex for the time being, although a crack at Quevega in the OLBG Mares' Hurdle at Cheltenham has been put on hold until next year at the earliest.
"She's not very quick, but there have been so many mares' races rained off we had to get her out and it just so happened this race was over two miles," said Henderson.
"She'd been running in staying races in France where she had 18 runs in 14 months so we gave her a break.
"There's a mares' Listed hurdle over two and a half miles at Sandown on Friday week (Jane Seymour Mares' Hurdle) that she'll go for next.
"As she's still a novice we don't have to send her for the David Nicholson (OLBG Mares' Hurdle) this year."
Minella Forfitness (11-4 joint-favourite) got the better of two former classy Flat performers.
Dual Cesarewitch winner Aaim To Prosper and November Handicap scorer Zuider Zee gave the winner something to aim at but he prevailed by two lengths.
"He was pretty keen and at halfway I thought I'd made a mistake stepping him up in trip," said Henderson.
"Barry said he wasn't as keen as he looked, though, and looking at the race you'd say the first three were much the best.
"He wants this trip but Aintree could be more his place than Cheltenham."
The Paul Nicholls-trained Ruben Cotter (85-40) made a winning chasing debut in the 32Red.com Novices Chase.
His main rival in the straight was the 6-4 favourite Vintage Star but the concession of 7lb took its toll and eight lengths separated them at the line.
Winning jockey Daryl Jacob said: "He did it nicely and travelled well the whole way.
"He's always looked a nice horse at home but disappointed on the track.
"He jumped well and when I got stuck into him he responded."
Be My Deputy (9-2) benefited from a positive Denis O'Regan ride in the 32Redpoker.com Handicap Chase, while Nick Scholfield successfully linked up with the Claire Dyson-trained Giveitachance (8-1) in the three-mile handicap hurdle.
The Fehily-ridden Overdante (16-1) survived a stewards' inquiry to win the 32RedMobile.com Handicap Hurdle, in which Tennessee Bird was beaten by the shortest of heads.