I know there are a few fics meeting the above,but I'm looking for a specific one.
Peter was one of Tony's servants. Tony stays behind so that his people escape, and he thinks he will be sacrificed. He is bought to the throne room, and is upset o see Peter and all the other people in the Kingdom celebrating - he thinks they are celebrating his death.
Peter eventuality realizes Tony doesn't understand they are celebrating his marriage to the conquering Lord.
I think it was Steve/Tony but I fell like I've searched everywhere in that paring so I may be wrong.
Edit -found in comments - The Red String, by masterlokisev159, on AO3