!! Warning: Whole answer contain major spoilers. Proceed with maximum caution.!!

The Hinokami Kagura dance is passed down through the Kamado family as a ritualistic dance performed at the start of every new year in the lunar calendar. It was passed down generation after generation along with the Hanafuda earrings. It was first utilized in combat by Tanjiro Kamado in his fight against Lower Rank Five Rui in the Mount Natagumo Arc. Hinokami Kagura was later revealed to be a descendent of the Sun breathing technique, the first and most powerful breathing style originally invented by legendary swordsman Yorichii Tsugikuni. Tsugikuni taught his friend Sumiyoshi Kamado the technique, which was passed down through the Kamado family and later renamed Hinokami Kagura. After Tanjiro's cleaning-up of some unnecessary moves that gradually accumulated through generations, it became the fully functioning and combat oriented original Sun breathing. The Sun breathing have 12 forms, all of which are powerful moves that can decapitate most demons. However, during his fight with Muzan, Tsugikuni discovered that any single Sun breathing move cannot kill Muzan, especially since Muzan cannot be killed just by decapitating. In order to defeat Muzan, Tsugikuni continuously performed all twelve forms of the Sun breathing technique in quick succession, enabling him to utilize the forms in a combined and most effective way. Requiring all twelve forms was mainly due to the fact that Muzan have twelve vital organs, namely seven hearts and five brains, that need to be all destroyed in quick succession (to prevent him from regenerating) in order to kill him. This combination of all twelve forms later became another form, the thirteenth from, or the "combined form". It has no specific name, thus it is referred to as the "thirteenth form". It is the most powerful form of all Sun breathing, and although Tsugikuni wasn't able to completely kill Muzan using the thirteenth form, he nevertheless destroyed 1500 of the 1800 pieces Muzan divided himself into after losing the upper hand, and inflicted wounds on Muzan that would never heal even with Muzan's super regeneration abilities. Thus, we can safely assume that although all Sun breathing forms are much more powerful than other breathing forms, the combined thirteenth form is the strongest of the strongest of all time. I believe that Tanjiro Kamado did actually use this form in his final battle with Muzan, although it wasn't explicitly stated so I'm not so sure.

!! Warning: Whole answer contain major spoilers. Proceed with maximum caution.!!

The Hinokami Kagura dance is passed down through the Kamado family as a ritualistic dance performed at the start of every new year in the lunar calendar. It was passed down generation after generation along with the Hanafuda earrings. It was first utilized in combat by Tanjiro Kamado in his fight against Lower Rank Five Rui in the Mount Natagumo Arc. Hinokami Kagura was later revealed to be a descendent of the Sun breathing technique, the first and most powerful breathing style originally invented by legendary swordsman Yorichii Tsugikuni. Tsugikuni taught his friend Sumiyoshi Kamado the technique, which was passed down through the Kamado family and later renamed Hinokami Kagura. After Tanjiro's cleaning-up of some unnecessary moves that gradually accumulated through generations, it became the fully functioning and combat oriented original Sun breathing. The Sun breathing have 12 forms, all of which are powerful moves that can decapitate most demons. However, during his fight with Muzan, Tsugikuni discovered that any single Sun breathing move cannot kill Muzan, especially since Muzan cannot be killed just by decapitating. In order to defeat Muzan, Tsugikuni continuously performed all twelve forms of the Sun breathing technique in quick succession, enabling him to utilize the forms in a combined and most effective way. Requiring all twelve forms was mainly due to the fact that Muzan have twelve vital organs, namely seven hearts and five brains, that need to be all destroyed in quick succession (to prevent him from regenerating) in order to kill him. This combination of all twelve forms later became another form, the thirteenth from, or the "combined form". It has no specific name, thus it is referred to as the "thirteenth form". It is the most powerful form of all Sun breathing, and although Tsugikuni wasn't able to completely kill Muzan using the thirteenth form, he nevertheless destroyed 1500 of the 1800 pieces Muzan divided himself into after losing the upper hand, and inflicted wounds on Muzan that would never heal even with Muzan's super regeneration abilities. Thus, we can safely assume that although all Sun breathing forms are much more powerful than other breathing forms, the combined thirteenth form is the strongest of the strongest of all time. I believe that Tanjiro Kamado did actually use this form in his final battle with Muzan, although it wasn't explicitly stated so I'm not so sure.

See Hinokami Kagura Wiki Fandom for more.