Which Hindu god or mantra is associated with the Ajna Chakra?

The presiding deity of the Ajna Chakra is Ardhanarishvara — the composite form of Shiva and Shakti representing the union of opposites at the third eye. Alternatively, Shiva alone is often depicted as the lord of the Ajna because his iconic third eye represents this centre. The goddess is Hakini Devi, described in the Tantras as a six-faced deity representing the six subtle qualities perfected at this chakra. The bija mantra (seed sound) of the Ajna Chakra is OM (ॐ) — considered the most powerful mantra in Hinduism. For specific Ajna chakra meditation, the mantra “OM Aing Namah” is used. The Gayatri mantra is also traditionally associated with Ajna because it invokes Savitri — the goddess of divine illumination that the Ajna perceives.