We first hear it called "Durin's Bane" in LOTR when Gimli says:
Soon afterwards:The Dwarves tell no tale; but even as mithril was the foundation of their wealth, so also it was their destruction: they delved too greedily and too deep, and disturbed that from which they fled, Durin's Bane. Of what they brought to light the Orcs have gathered nearly all, and given it in tribute to Sauron, who covets it.
Celeborn afterwards said:
But 220 years earlier:'Alas! ' said Celeborn. `We long have feared that under Caradhras a terror slept.
So why was everybody so surprised? After all Dain was the leader of the Dwarfs in the North, not a mythological character that nobody could talk with. Somebody might have debriefed him.Up the steps after him leaped a Dwarf with a red axe. It was Dáin Ironfoot, Náin's son. Right before the doors he caught Azog, and there he slew him, and hewed off his head. That was held a great feat, for Dáin was then only a stripling in the reckoning of the Dwarves. But long life and many battles lay before him, until old but unbowed he fell at last in the War of the Ring. Yet hardy and full of wrath as he was, it is said that when he came down from the Gate he looked grey in the face, as one who has felt great fear...
Then Thráin turned to Dáin, and said: 'But surely my own kin will not desert me?' 'No,' said Dáin. 'You are the father of our Folk, and we have bled for you, and will again. But we will not enter Khazad-dûm. You will not enter Khazad-dûm. Only I have looked through the shadow of the Gate. Beyond the shadow it waits for you still: Durin's Bane. The world must change and some other power than ours must come before Durin's Folk walk again in Moria.
"Dain did not give leave willingly" is rather a mild disapproval from someone who should know better.Glóin sighed. `Moria! Moria! Wonder of the Northern world! Too deep we delved there, and woke the nameless fear. Long have its vast mansions lain empty since the children of Durin fled. But now we spoke of it again with longing, and yet with dread; for no dwarf has dared to pass the doors of Khazad-dûm for many lives of kings, save Thrór only, and he perished. At last, however, Balin listened to the whispers, and resolved to go; and though Dáin did not give leave willingly, he took with him Ori and Óin and many of our folk, and they went away south.