To restore the poem for Nurnen’s Ithildin Door the correct words must go in the correct slots. The first word is iron, the second serpents, the third avail, the fourth woe, the fifth hordes and the last claim:

To restore the poem to open the Ithildin Door the order for the words is iron, serpents, avail, woe, hordes and claim.
Reach Nurnen Barrows
All Ithildin Door Guides
All Shadow of War Guides