This is the 16632nd most frequent Russian word.
сепаратистов
"Separatists."
Here, "сепаратистов" functions as the object of the preposition "о" and is used in the genitive plural form to express the group of people being discussed.
В новостях сообщили о деятельности сепаратистов.
The news reported on the activities of the separatists.
In this sentence, "сепаратистов" is used in the accusative plural form as the direct object of the verb "предприняло," depicting the target of the measures.
Государство предприняло меры против сепаратистов.
The state took measures against the separatists.
"Сепаратистов" is in the genitive plural form here, being the object of the preposition "об," indicating the possessive relationship for whose ideas are being described.