"A sword's poor company for a long road." —Yasuo
Ancient Sword is an item dropped by the Umbra Boss, with a guaranteed drop. After getting this item from the boss, talk to the Umbra NPC, and he'll give you Yasuo only if you have the sword. You may or may not trade for this if you don't have the first part of the quest given by the NPC completed. Some trade for this just to save it and have it ready after completing the final quest given by the NPC.