There is a chapter about that ritual in Lopez, Donald S. "Elaborations on Emptiness: Uses of the Heart Sūtra". Princeton University Press, 1996The Heart Sutra is often recited together with a supplemental section for dokpa, the practice of averting harm and negativity. The text of the dokpa section refers to an incident recounted in the Perfection of Wisdom Sutra in Eight Thousand Lines and Perfection of Wisdom Sutra in Eighteen Thousand Lines, when the god Indra turned away Mara and his forces, who were approaching the Buddha, by contemplating and reciting the Prajnaparamita.
Protectors of the Prajnaparamita sutra according to the Japanese tradition :
http://www.onmarkproductions.com/html/s ... orsofShaka
Sanskrit names:16 Good Gods
(Jūroku Zenjin, Juroku Zenjin 十六善神). Protectors of the Daihannyakyō Sutra 大般若経 (Great Widsom Sutra, Skt. Mahaprajna paramita sutra) and those devoted to it.
Daitorada 提頭羅宅; Jikokuten 持国天 (one of four Shitennō)
Birurokusha 毘盧勒叉; Zōchōten 増長天 (one of four Shitennō)
Ueshiramanu 吠室羅摩拏; Tamonten 多聞天 (one of four Shitennō)
Birubakusha 毘盧博叉; Kōmokuten 広目天 (one of four Shitennō)
Bonten 梵天 and Taishakuten 帝釈天 are also sometimes added, making eighteen.