Glad to see Riddles is active again,
Hmm Legate, I think I might have it...
We left our homes - Ered Luin, 21st April, 100 years ago last Thursday
for the new lands. - Erebor
With my high hopes - for gold and revenge upon Smaug
I failed my friends. - wandered from their camp
Rain pouring down, - a 'black rain'
creature unknown - yup, still unknown
that brought us doom. - well, me anyway
Now I am left to talk to myself. - my companions and RING taken from me
What now to do? Sit down or play? - witless and wandering, map-and-key games?
But the doom came: Forever stay. - in the dungeons of the Necromancer
So that means its Thrain (the Second if we're being pedantic)
Cheers