[Iarwain]

I'm going to guess that it is the Witchking's ring of power.

In the beginning, I knew it!- I knew where it was (on his finger)
Now I am all confused- lost/dissintigrated during his fall
I lost the one thing I know
A Hero of old had used- I believe he would have been considered a hero very much by the people of Angmar

I saw an old king falling- Eowyn fells the beast
Greed was on his tongue- obviously
A woman on the verge of lost- Eowyn indeed
Now stricken, on the run- the ride of the Rohirrim
Radiant shines the hope of all- the darkness breaks, giving sunlight to the Pellenor
Shadows hover low- witchking falls
Tell me, friend, what have I lost

If, indeed, you know....

There are possible variations on the meaning, but I think this works. Depending on how literally the word radiant is to be taken, "Radiant shines the hope of all", combined with "shadows hover low" could be Merry on the ground.

Iarwain