Mai has the special ability to create items. However, mai has a limited ability in creating items and knows that creating bigger and difficult items will put a strain on herself and her health. In this chapter mai sacrificed herself to create a cursed weapon for maki to use. Mai has also figured out that neither mai or maki will ever be able to reach their full potential if the other exists since twins share power and are considered one individual.
Basically Mai sacrificed herself to make Maki the ultimate cursed weapon (this what she hands Maki) and release Maki's full power and potential.
Jesus. Every time I read a new chapter I get more confused lol. Did they just become like one being or something?