"He" is a DNA supercomputer on Bourak. The Hassassins use its computing power to predict where and when targeted, small-scale violent interventions will have the most effect for keeping Haqqislam safe.
While the Hassassin AI seems poetic (Aleph, the EI, Svengali and TOMotM duking it out in 5D Chess is such a cyberpunk concept), I see the Old Man on the Mountain as Infinity's version of Ras Al Ghul.
I rather like the idea that they're the digitized consciousness of the Hassasins best analysts in a sped up virtual environment, lets them process information at an apparently accelerated rate. ... I may or may not be rereading the Culture series...
