The gift the human Emperor gave in return to the Sea Emperor was a completely polished Dragon-slaying Sword.
The Sea Clan naturally has disadvantages in metal processing, so they need to order a lot of their weapons from humans or dwarves. And due to the corrosion of seawater, ordinary weapons are extremely difficult to preserve in the sea for long, so the Sea Clan often has to replace their weapons. It can be said that the biggest aspect of sea-human trade lies in metal products.
This completely polished and corrosion-protected Dragon-slaying Sword was a gift the Emperor had painstakingly prepared. It was custom-made from the Golden Hammer, representing the highest level of craftsmanship in the land world. Notably, the dwarves' favorite flat and straight style was fanatically applied to this sword.