Note: The structure of this riddle is modeled after those written in old English, where alliteration takes center stage just as much as rhyme.
I am a giant, an ancient king
With many a jewel by my neck are hanging
Then a monster comes, a heartless thief.
The bite so bitter brings me grief.
It takes my treasure, my riches untold,
My rubies, my emeralds, my silver, my gold.
What am I?