This is the 8251st most frequent Russian word.
мардакертского
"Of or relating to Martakert," referring to the region or town in Azerbaijan/Artsakh.
The word 'мардакертского' is used as an adjective derived from 'Мартакерт', denoting something related to Mardakert, modified by the genitive case to describe 'района' (region).
Он изучает историю мардакертского района.
He is studying the history of the Mardakert region.
Here, 'мардакертского' describes the cathedral, showcasing its affiliation with Mardakert. The usage is in the masculine singular form to match 'собора'.
Я был поражен архитектурой мардакертского собора.
I was impressed by the architecture of the Mardakert cathedral.
In this context, 'мардакертского' is used as an adjective to indicate the origin of the sample being from Mardakert.