Mordekaiser's helm.
https://vignette1.wikia.nocookie.net/leagueoflegends/images/e/e9/Noxus_Crest.png/revision/latest?cb=20161117055949
Noxus's citadel was built on the remains of Mordekaiser's bastion. Noxus' crest is some mix between an axe and a helm. It woud make sense to think it was inspired by Mordekaiser's helm. Make reworked Mordekaiser's helm very similar to the one in Noxus' crest please! :)