To open a Ithlidin Door, you need to find all Itlhildin parts of the poem in that region. In Minas Ithil, there are six and they will appear on your map when you purge a Haedir point, by using the eye to locate the collectables. If you need to find the symbol, open up the map (
) and press
to see a legend.
Once you have collected all 6, head to the door, which also appears on your map, to the south, underground in the Barrows. You can fast travel to this area once you have visited it during the story in Act I.
The poem has six words missing and you need to place the words in the correct order to complete the poem. It should read as follows:
In the land of Mordor where the Shadows lie
We bear our banners aloft, boots like Drums
Our Cadence quick, our countenance dire.
For who in this dark, dream-haunted Land dares
Resist the righteous flame of Wrath
And Doom themselves to despair and death?