Muzan's tentacle-like appendages are highly toxic. When Muzan cut someone with it, members of the Demon Slayers began to experience the effects of the poison, weakening them gradually until an antidote was delivered and administered during the middle of their their battle against Muzan outside of Infinity Castle.
Just a tidbit.
Heat based weapons slow down Muzan's regenerative capabilities, and Sunlight is lethal for him.