हूं (hūṃ) in Dvenagari, (吽) in Japanese.
This is the best I can get. It is not in Unicode, I don't know if there is a version that is. If you make a raster image for it, like with the DigiLetters, it would be so great if you could make a black version for the Digimon Wiki, too. The visiblemantra site is probably the best bet for finding the Deva Siddham, too.
In Om mani padme hum, it means Wisdom, its samsara realm is Naraka, its color is black.