The demon that appeared before Tanjiro and Rengoku after they defeated Enmu was the Upper Three, Akaza. While injured and on the ground, Tanjiro is almost attacked by Akaza and narrowly escapes death when Rengoku quickly strikes back. During the intense battle between Rengoku and Akaza, Akaza suggests to Rengoku that he should also become a demon. Rengoku vehemently refuses Akaza's offer, but his injured body is exhausted and being pushed past its physical limits...
There should be some notes there
left by the preceding Flame Hashira.
Tenemos apuntes
de todos los Pilares de Fuego.
Allí guardamos las notas
de los antiguos Pilares de las Llamas.
Deve haver anotações dos Pilares 
das Chamas ao longo da história.