Dia just dedicated the rest of her existence to Sig.
With Sig recognizing her will to exist, the trial they just overcame together and with the significant psychological impact which losing her dominant hand landed on her, this made her fixate/envelope her 'purpose in life' all around Sig.
I mean, she no longer has a hand to use a sword anymore, but she still has a overpowered divine magic and she knows that Sig is the only who ever treated her as normal living being - all of this factors now made her be the potential yandere that will only use her powers 'freely' and with all her might, as long as it is for Sig. It seems that the other girl from the earlier chapters will have to really work hard to get Sig now.