oṃ sarvatathāgatoṣṇīṣasitātapatre hūṃ phaṭ hūṃ mama hūṃ ni svāhāIf we break the sandhi and add dashes for compounds it reads:
uṣṇīṣa-sita-ātapatre hūṃ phaṭ hūṃ mama hūṃ ni svāhā
So now it becomes clear that this is a mantra for Uṣṇīṣa-sitātapatra (also spelt Uṣṇīṣa-sitātapattra) a Tantric goddess with 1000 heads, 1000 arms, and 1000 legs! Her name means something like 'crowned with a white parasol'. Her iconography is very complex, though apart from her multiple limbs she does carry a white (sita) umbrella/parasol (ātapatra) or sometimes a Tibetan victory banner. Robert Thurman tells us that she is a special form of Tārā and a counter part to the 100o armed form of Avalokiteśvara (The Sacred Art of Tibet p.319-321).
Here's the mantra written out in dbu-can.