A curious reference to a khrims rnga, a drum of the law, appears in the rLangs kyi po ti bse ru rgyas pa. This semi-mythical genealogy of the rLangs clan was probably compiled in the late fourteenth century, quite possibly from earlier sources. In an early section, the sage Jangchub Drékol (Byang chub ’dre bkol) (c. 11th century) travels to eastern Tibet in search of his destined disciples. There he meets Ling Gesar, who gives the him various gifts. These include, [p. 46]:
བདག་ལ་ཁྲིམས་ཀྱི་དངུལ་མདུང་ཀེ་རུ་དང་། ཁྲིམས་ཀྱི་རྔ་བོ་ཆེ་ཟིལ་གནོན་དང་། ཁྲིམས་དར་ནག་པོ་དགྲ་འདུལ་དང་།
The silver spear of the law [called] Ke ru, the great drum of the law [called] the vanquisher, and the black banner of the law [called] the subduer.
The significance of these artefacts is not entirely clear, but the drum and banner are clearly symbols of power, and the implication is that their force is shamanic.
In other texts, the khrims rnga has more prosaic associations. The Padma bka’ thang, a biography of Padmasambhava created by the treasure-revealer Urgyen Lingpa (O rgyan gling pa) in around 1352, contains prophetic passages (chapter 92). It refers to a time when:
The army will beat the drum of the law at Bya rog fortress;
And faint-hearted men will throw gold dust into the river.
This is a reference to an incident in the 1280s, when the emperor Qubilai dispatched an army to Tibet to threaten the Sakya dpon chen. Here, the drum of the law seems to be a straightforward symbol of the imperial army. In a similar way, the phrase khrims dmag, ‘army of the law’, is used by Urgyen Lingpa and also by Jangchub Gyaltsen (Byang chub rgyal mtshan), in his testament, to refer to the imperial army. The word khrims, that is, indicates imperial power and authority and the army uses a drum, presumably to announce its arrival.
In later centuries, a drum was also used to announce the proclamation of government edicts and in the law courts. Alexander Cunningham refers to the khrims rnga at the court (the shags khang) in Ladakh, in the account of his travels in the mid-nineteenth century. In his dictionary, Jaeschke cites the phrase khrims kyi rnga bo che brdungs te, from the Tibetan rGyal rabs, and translates it as ‘the beat of the large proclamation (or law) drum’ (p. 133). Das translates the same phrase as ‘having beaten the large drum for the government edicts’, (p. 367).
It seems that the symbol of the shamanic power of Ling Gesar, which appears in the Lang Potiseru and which might date from the early medieval period, becomes a more straightforward symbol of political power in the Yuan administration and within the governmental structures of the Ganden Podrang and the Ladakhi kingdom.
It is tempting to assume that the ideas of a mythical and idealistic account will tend to take on more prosaic meanings as political institutions develop stronger and more secular structures. However, reflecting on the role of the drum in Tibetan culture, more generally, A. M. Trewin has suggested that its musical roles, the lha rnga, were a means by which a king could gain access to the transcendental realm. They were a way to attain legitimacy in the eyes of the people. The lha rnga plays a mediating role, he says, across a transcendental-worldly axis.
In the case of the khrims rnga, then, it is also possible that the movement was in the opposite direction. A prosaic symbol of the Yuan administration, used by Urgyen Lingpa, becomes an image with transcendental connotations in the Potiseru. This text was probably only compiled in its final form in the fourteenth century, after all. This imagery was, then, valued and retained by later political regimes for its associations with the transcendent.
 The extracts here can be found in historical documents section of this website.
 This incident is described in the Gung thang dkar chag. See Per Sørensen and Guntram Hazod, Rulers on the celestial plain: ecclesiastic and secular hegemony in medieval Tibet (Wien: Verlag der Österreichischen Akademie der Wissenschaften, 2007), 185–87.
 A. M. Trewin. Rhythms of the gods: the musical symbolics of power and authority in the Tibetan Buddhist kingdom of Ladakh. Ph.D. thesis (City University), p. 253.