The Iron Shackle artifact can be found at the Black Gate, in the northern part of Udûn.
Iron Shackle, Second Age, circa 3436
"These are prisoner's shackles, leg irons to be specific. They date back to the Second Age and were most likely worn by Gondorian captives, men and women whose involuntary servitude built the Towers of the Teeth flanking the Morannon.
"All right, worms. Welcome to The Black Gate. You'll be building our watchtowers for us. Hard work will be rewarded.
Sir, do you mean hard work will be rewarded with a shorter sentence?
You're traitors, the lot of you. Murderers and thieves. When I say rewarded, I mean I won't throw you to the Caragors."