Muzan's "aura" wasn't actually aura, muzan was constantly getting burned by the wisteria flowers that they used as a barrier against the demons, but since muzan is incredibly powerful it does not affect him in any way, he keeps regenerating...

Muzan had nothing but aura in that walking sequence. Believe it or not, none of that walking was in the manga. The chapter just ends off with Muzan appearing in front of the master, so ufotable absolutely cooked with that last scene. I assume ufotable is gonna stick with a three-movie format for the final arc (at least that’s what’s known rn). However they decide to do it, it’ll be nothing but straight heat and action. Let’s hope this season finale will be a good one! I’m looking forward to that reaction.

We need Tanjiro there ASAP