Siddhaṃ (Xītán 悉曇)
Siddhaṃ is a South Asian alphabet that was used to write Sanskrit texts in North India during the period 600-1200 AD. When Buddhism dwindled in India, the use of Siddhaṃ was passed on to China with Mahāyāna and Vajrayāna Buddhist texts along the Silk Road. As a result, Siddhaṃ script is the only South Asian alphabet that is only used in East Asia and not in South Asia.
Mahāyāna Buddhism considers that the meaning of a text is of paramount importance, hence the great deal of translation work conducted under the Táng in China. On the contrary, Esoteric Buddhism places a lot of emphasis on the sounds contained in a given text, considering that they carry a hidden meaning that would be ruined by the translation. As Chinese characters are not suitable for writing the sounds of Sanskrit, the Siddhaṃ script has been preserved in Tantric Buddhism to conserve the original pronunciation of mantra.
In gaming terms, a calligraphed Siddhaṃ syllable plays exactly the same role as a mandala: it can be used to replace a mudrā while casting a spell, and also as a focus to store power points, by members of the Mìjiào 密教 and Mìzǒng 密宗 sects.
Edit (7 May 2013): I have found an extremely interesting and lengthy article about Siddhaṃ on the Sino-Platonic Papers scholarly site: Siddham in China and Japan. Enjoy!