Ringil was the sword of Fingolfin, High King of the Noldor (the Noldor are
one of the three branches of high-elves in Tolkien's mythologies, the ones
who suffered most from the corruption of Morgoth). Fingolfin is perhaps
one of the greatest heroes in Tolkien. He fought his way basically
singlehandedly to the gates of Angband itself and shouted his challenge to
Morgoth at the walls. Morgoth came out of Angband to engage him in single
combat (the last time Morgoth ever left Angband until he was finally
defeated by the Valar).
Fingolfin, a mere elf, not even a Maia, took on Morgoth, the most powerful
of all of the Valar, in single combat with Ringil and wounded him sorely
seven times, wounds that hurt Morgoth for the remainder of his physical
existence. Morgoth himself wielded Grond against him, and after a truly
epic battle, Fingolfin fell to Morgoth and was killed.
The basis of Ringil's power is in the tale of the battle, where Fingolfin
repeatedly avoided the blows of Grond by dancing aside and then striking
back (high speed). The sword is definitely of sufficient importance in
Tolkien to warrant its position as the most powerful weapon in the game.
For more details, see the _Silmarillion_ (commonly called the Sil).
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