That feudal lord rules over a domain of one hundred thousand koku.
Literal
That feudal-lord [topic-は] 100,000-koku [obj-を] ruling-[progressive-ている].
大名 'feudal lord / daimyō' is the standard term. 万石 (まんごく) — 石 (こく) is the historical unit of rice yield used to measure a domain's wealth. 領する 'to rule over / govern' is a formal/literary verb. A historical-vocabulary sentence.