necessitas

Necessitas, necessitatis is third declension feminine, one of the large family of abstract nouns in -tas, -tatis built off adjectives: from necesse comes necessitas the way veritas comes from verus and libertas from liber. They are all feminine, they all take -tatem in the accusative, and they all keep the t through the oblique cases.

The ablative does most of the work. Necessitate coactus, "driven by necessity", is a set phrase, and ex necessitate is the standard "of necessity" of scholastic argument. Watch the register: classical necessitas is often grim, the compulsion of fate or hunger, while medieval writers use it flatly for a practical need. Necessitas legem non habet, "necessity has no law", is a canon-law maxim rather than a classical tag.