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...
I'll damage his body
as much as I can in an instant!
Cercenaré un buen pedazo de su cuerpo.
Voy a cercenarle
un buen pedazo del cuerpo.
Eu vou ter que arrancar as ervas 
daninhas, todas de uma vez…