The Graves of Kings and Heroes (and Giants) in the Frostfangs (Spoilers Extended)

While supposedly looking for the Horn of Winter for his upcoming invasion the wildlings supposedly opened numerous graves.

"Not for fear!" She kicked savagely at the ice beneath her with a heel, chopping out a chunk. "I'm crying because we never found the Horn of Winter. We opened half a hundred graves and let all those shades loose in the world, and never found the Horn of Joramun to bring this cold thing down!" -ASOS, Jon IV

and:

"They all do," said Jon. "Ygritte said they opened a hundred graves . . . graves of kings and heroes, all over the valley of the Milkwater, but they never . . ." -ASOS, Jon VI

So what I started to wonder was whose graves did they open? Which heroes/kings?

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The wildlings are referred to as heroes numerous times

They refer to themselves, others refer to them as heroes, etc.

Jon to Ygritte:

Wildlings fought like heroes or demons, depending on who you talked to, but it came down to the same thing in the end. They fight with reckless courage, every man out for glory. "I don't doubt that you're all very brave, but when it comes to battle, discipline beats valor every time. In the end Mance will fail as all the Kings-beyond-the-Wall have failed before him. And when he does, you'll die. All of you." -ASOS, Jon V

Val:

Val had reminded him of that, on his last visit with her. "Free folk and kneelers are more alike than not, Jon Snow. Men are men and women women, no matter which side of the Wall we were born on. Good men and bad, heroes and villains, men of honor, liars, cravens, brutes … we have plenty, as do you." -ADWD, Jon V

Thoren Smallwood:

"I know them as well as you do, Buckwell," Thoren Smallwood snapped back. "And I mean to have their heads, every one. These are wildlings. No soldiers. A few hundred heroes, drunk most like, amidst a great horde of women, children, and thralls. We will sweep over them and send them howling back to their hovels. -ACOK, Jon V

But it should also be noted: The Wildlings burn their dead:

When he brought the skull to Mormont, the Old Bear lifted it in both hands and stared into the empty sockets. "The wildlings burn their dead. We've always known that. Now I wished I'd asked them why, when there were still a few around to ask."

Jon Snow remembered the wight rising, its eyes shining blue in the pale dead face. He knew why, he was certain.

"Would that bones could talk," the Old Bear grumbled. "This fellow could tell us much. How he died. Who burned him, and why. Where the wildlings have gone." He sighed. "The children of the forest could speak to the dead, it's said. But I can't." He tossed the skull back into the mouth of the tree, where it landed with a puff of fine ash. "Go through all these houses. Giant, get to the top of this tree, have a look. I'll have the hounds brought up too. Perchance this time the trail will be fresher." His tone did not suggest that he held out much hope of the last.

Two men went through each house, to make certain nothing was missed. Jon was paired with dour Eddison Tollett, a squire grey of hair and thin as a pike, whom the other brothers called Dolorous Edd. "Bad enough when the dead come walking," he said to Jon as they crossed the village, "now the Old Bear wants them talking as well? No good will come of that, I'll warrant. And who's to say the bones wouldn't lie? Why should death make a man truthful, or even clever? The dead are likely dull fellows, full of tedious complaints—the ground's too cold, my gravestone should be larger, why does he get more worms than I do . . ."-ACOK, Jon II

and:

By late afternoon the snow was falling steadily, but the river of wildlings had dwindled to a stream. Columns of smoke rose from the trees where their camp had been. "Toregg," Tormund explained. "Burning the dead. Always some who go to sleep and don't wake up. You find them in their tents, them as have tents, curled up and froze. Toregg knows what to do." -ADWD, Jon XII

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So the wildlings opened graves in the frostfangs belonging to heroes/kings, but the wildlings custom is to burn their dead. So that means that the wildlings are obviously Valyrian/secret targs are opening graves from before the long night or their are some cultures that don't follow this death process.

It should be also noted that they looked in a giant's grave as well:

"Did she?" Tormund slapped his thigh and hooted. "She burned that fine big horn, aye. A bloody sin, I call it. A thousand years old, that was. We found it in a giant's grave, and no man o' us had ever seen a horn so big. That must have been why Mance got the notion to tell you it were Joramun's. He wanted you crows to think he had it in his power to blow your bloody Wall down about your knees. But we never found the true horn, not for all our digging. If we had, every kneeler in your Seven Kingdoms would have chunks o' ice to cool his wine all summer." -ADWD, Jon XII

So if we put all this together:

• Mance is up in the Frostfangs (out of the way for what he is trying to accomplish)

• Searching for "some power" (Horn of Winter), how he knew to look there is great question

• Opened numerous graves (kings/hero/giants)

• Possibly let "shades loose into the earth" (although this contradicts with Mance's supposed reason for going south, although some of his followers have different goals/agendas)

• Wildlings burn their dead

So my guess is that it could involve the Long Night/Last Hero:

How the Long Night came to an end is a matter of legend, as all such matters of the distant past have become. In the North, they tell of a last hero who sought out the intercession of the children of the forest, his companions abandoning him or dying one by one as they faced ravenous giants, cold servants, and the Others themselves. Alone he finally reached the children, despite the efforts of the white walkers, and all the tales agree this was a turning point. Thanks to the children, the first men of the Night's Watch banded together and were able to fight—and win—the Battle for the Dawn: the last battle that broke the endless winter and sent the Others fleeing to the icy north. Now, six thousand years later (or eight thousand as True History puts forward), the Wall made to defend the realms of men is still manned by the sworn brothers of the Night's Watch, and neither the Others nor the children have been seen in many centuries. -TWOIAF, Ancient History: The Long Night

But the letting shades loose is definitely paralleled by the crypts of winterfell:

By ancient custom an iron longsword had been laid across the lap of each who had been Lord of Winterfell, to keep the vengeful spirits in their crypts. The oldest had long ago rusted away to nothing, leaving only a few red stains where the metal had rested on stone. Ned wondered if that meant those ghosts were free to roam the castle now. -AGOT, Eddard I

and:

He was trapped here, with the ghosts. The old ghosts from the crypts and the younger ones that he had made himself, Mikken and Farlen, Gynir Rednose, Aggar, Gelmarr the Grim, the miller's wife from Acorn Water and her two young sons, and all the rest. My work. My ghosts. They are all here, and they are angry. He thought of the crypts and those missing swords. -ADWD, A Ghost in Winterfell

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I had a few more quotes I was going to include but they tended to focus on "some power"/the horn and it seemed the post was turning more into a discussion of them looking for that instead of what could have occurred that left bodies/graves up there and that Mance knew to go there and look.

TLDR: Since the wildlings burn their dead, a battle or massacre, etc. must have occurred before they began to do so (assuming as a result of the Long Night) up in the valley of the Milkwater/Frostfangs