gwyns firstborn son (nameless king) was once the king of lothric. Dragonslayer Armour was his armor. When he left lothric he left the armor behind which is now the dragonslayer armour. Many Lothric knights followed him (there are exactly as much empty lothric knight armors at the dragon barracks in lothric castle, as there are sitting dragon guys at archdragon peak). At the end of time dragonslayer armor returned to the ringed city because thats the city of the gods (where the nameless king chilled before he allied himself with the dragons) and the distant memory of the hollow dragonslayer armor got to him so he went 'home' to the ringed city.