The escape story exists in German songs (The Dietrich Cycle), according to Wikipedia, that do not mention a marvellous necklace. However, the necklace may be familiar, too. Its name--Brosingamen--is very similar to Brisingamen (the Bright or Flaming Ornament), the necklace that belonged to the goddess Freyja. It features in two major sources of Icelandic mythology: The Poetic Edda and the Prose Edda. It is a good guess that both names refer to one mythological object.
Beowulf's neckace will go to his King, Hygelac, and he will die in battle wearing it. It will be stripped from him, along with his armour.
After giving the history of the gift, the poet returns us to the hall of Heort, which is roaring with approval of Wealhtheow's gift. She then says a few fitting words: please accept this gift, remain a friend and a support to my two sons, and (playing to the crowd) that this hall has loyal and true men guarding it. Her anxiety about the fate of her children is expressed again, as it was in her earlier words to Hrothgar.
When the revelry concludes, the warriors prepare to sleep. Another recurring theme of the poem is expressed here. The men can relax because they are not aware that their fate looms over them as they sleep, oblivious. One, in particular, is "happy and doomed." In fact "to sleep after the feast" is a common euphemism in Beowulf for death.
XIX. BEOWULF’S NECKLACE AND HEROES’ REST
He was carried a cup and kindly words
were spoken in greeting, spirals of gold
graciously proffered, a pair of arm-bands,
tunic and rings and a torc, larger
than any other on Earth I know of.
I heard of no better under heaven’s arch
from a heroes’ hoard, since Hama brought
the Brosings’ torc to the bright tower,
both stone and setting. He escaped the connivings 1200
of Eormenric for eternal bliss.
Hygelac of the Geats, the grandson of Swerting,
in his last battle bore that necklace
when, under his flag, he defended wealth
won from the war-dead. Wyrd took him then,
after pride led him to look for sorrow,
a war with the Frisians. He had worn the gem,
the strange stone, over the stormy basin.
The strong sovereign stumbled under his shield.
The king’s corpse then was captured by Franks 1210
with the ornament and armour he wore.
Lesser warriors looted the dead
after battle’s bloodshed. The bodies of Geats
held the death-field.
The hall resounded.
Watched by the soldiers, Wealhtheow said,
“Enjoy this gem, gentle Beowulf,
“with warmest wishes, and wear these clothes
“and wealth from our land. Live up to your worth,
“demonstrate strength, and stay, to these boys,
“a kind teacher. I will take it to heart. 1220
“You have earned such fame that far and near,
“for time out of mind, men will praise you
“even as widely as the ocean holds
“the walls of land that winds inhabit.
“My prince, I wish you wealth in plenty
“and every success. Aid both my sons,
“a friend in their deeds, so fortune will smile.
“Every earl here honours the rest,
“contented and kind in the king’s protection.
“The soldiers are loyal; the servants, alert; 1230
“mellow with drink, the men do my bidding.”
She went to her seat. There was a feast.
The warriors drank wine, unaware of their fate,
a dire destiny, a doom that they shared
with countless earls.
When evening came
and Hrothgar had left to lie in his chambers,
the ruler to rest, the room was guarded
by a horde of men, as it had been often.
They carried off benches and covered the floor
with bedding and bolsters. One beer drinker 1240
lay in his hall-bed, happy and doomed.
They set at their heads their hard-bossed shields,
bright linden-wood. There lay on the bench
over each noble, easily noticed,
a lofty helmet, a linked byrnie,
a great-shafted war-spear. It was their custom
to always be battle-ready
whether at home or harrying abroad
for their sovereign king on such occasions
as troubles came. They were trusty men. 1250