Dagon's Domain Expansion is conjured when Dagon forms a seal on his stomach. This creates a tropical beach that has a forest of palm trees on one side of the shore and a vast ocean on the other side.
Horizon of the Captivating Skandha has a sure-hit technique that allows the shikigami that Dagon creates to automatically hit their target. This Domain Expansion has been described as a calm Domain.
Dagon used his Domain Expansion first time within an apartment to relax in the water while Mahito was sitting on the beach. Kenjaku commented that Dagon has quite the calm domain. Later, this Domain has been used frequently for similar purposes.
Later, after becoming a Cursed Spirit he utilized his Domain again for battle purpose against Kento Nanami, Maki Zenin, and Naobito Zenin. He used his Shikigami to attack them. However, Megumi broke into his Domain by expanding his Domain which lead to a battle of Domains and negating Dagon's sure-hit technique.