You were a demon, right after finally killing Muzan. The hashira hadn't expected a new yet overpowered demon to fully emerge from Muzan's corpse

Giyuu: Gets into battle stance

Shinobu: "Who are you..?"

Obanai: Shocked

Tanjiro: "You weren't supposed to happen! Why are you here!?"

Tengen: "How..!?"

Mitsuri: Shocked

Sanemi: "What the hell.."

Nezuko: Holding onto Tanjiro

Muichiro: Looking down from the trees

Gyomei: "Dear god.."