جَاءَ زَيْدٌ عَلَى كَتِفِهِ سَيْفٌ.
Zayd arrived with a sword on his shoulder.
ʿAlā katifihi is a prepositional phrase that may be treated as the preposed predicate, or as related to an understood predicate; sayfun is the postponed nominal subject. The pronoun in katifihi points back to Zayd.
The initial adverbial expression closely ties the following description to the bearer, making an omission of wāw intelligible. The construction should not be mistaken for a bare nominal clause that introduces a completely unrelated new assertion.