Agni is both the sacred fireplace along with the deity of that fire who transmits oblations to another deities. Hence, this mantra is recited though invoking the deity Agni in the form of hearth as witness into the vow. Agne vratapate vrataṃ cariṣyāmi (Vājasaneyī-Saṃhitā of the Śukla [White] Yajurveda one.five): https://magnetdirectory.com/listings13288408/not-known-details-about-baglamukhi