Nicky Henderson's second favourite for the feature race on the opening day of next month's Cheltenham Festival stepped at the final hurdle of the 32Red Hurdle and tumbled over it.
Both Darlan and champion jockey Tony McCoy were flung to the turf and the horse had to be destroyed.
JP McManus's charge was among the top performers in the novice ranks last term and won the Christmas Hurdle at Kempton on Boxing Day.
That saw the six-year-old priced at around 7/2 for the most prestigious hurdling event in the National Hunt calendar behind ante-post favourite Hurricane Fly.
Henderson said: "It's some game, isn't it? Why is it always the good ones."
Reigning Champion Hurdler Rock On Ruby held off Countrywide Flame to win the Listed two mile contest.