What does the Horn of Passage (found in the Quagmire I believe) do? I didn't find a use for it by the end of the game, so is it good for anything or is it just a red herring?

It opens doors. there is a two doors in the Iron Titan, basically you can only open one with a pressure plate, and from within u can get some weak treasure. but if you go back and use the Horn it will actually open the other door, for completion. but ultimately in the last Warlords keep there are a few doors that you might get stuck to open, and that will help, be carful though it only have like 3 charges.