These zoologists studying denning wolves witnessed that first hand.
While studding the behavior of denning wolves in the Naliboki Forest, a trail cam managed to capture a brown bear stumbling upon a den.
The den contained a number of very young wolf pups, AKA dinner for the bear.
According to the zoologists monitoring the cam, the bear made short work of that den:
“The brown bear discovered the wolf den and began destroying it. Extirpation of that by the bear took half an hour. It looks like the parent wolves were observing the bear’s action from aside, but they were afraid to approach the bear and attack it.
Right after the bear has gone the parent wolves came to the extirpated den. They looked scared. After some shy and careful inspection of the den-site, the parent wolves left the place and came back there only at night.
Also, they returned to the den-site few more nights, tried to dig and investigated the burrow-den inside and searched for their pups. Plausibly, the pups were killed and, perhaps, eaten by the bear.”
Because the motion-detecting camera didn’t capture the bear down in the den, you don’t get to see the carnage (probably for the best), but it’s safe to say that none of the pups survived the attack.
When the adult wolves returned, they didn’t find anything but a home in ruins.