Mantra for Ashtami Havan should be the one associated with any of the powerful forms of Goddess Durga. Below is a mantra for Ashtami Havan and it is associated with Goddess Mahagauri form of Mother Goddess Durga.
The mantra is to be chanted after the havan is set and the light has been lit. Please note that the mantra should be chanted with utmost dedication. Mantras should not be simply used.
Mantra For Ashtami Havan
सर्वसंकट हंत्री त्वंहि धन
ऐश्वर्य प्रदायनीम्।
ज्ञानदा चतुर्वेदमयी महागौरी प्रणमाभ्यहम्॥
सुख शान्तिदात्री धन धान्य
प्रदीयनीम्।
डमरूवाद्य प्रिया अद्या महागौरी
प्रणमाभ्यहम्॥
त्रैलोक्यमंगल
त्वंहि तापत्रय हारिणीम्।
वददं चैतन्यमयी महागौरी
प्रणमाम्यहम्॥