Why is it die Trilogie?

Trilogie ends in -ie, and nouns ending in -ie are feminine: in a German dictionary, 98% of them take die.

All nouns ending in -ie
Sense 1
die

Meaning

[1] Artistic work, which consists of three independent parts

[1] künstlerisches Werk, welches aus drei selbstständigen Teilen besteht

Examples

[1] The author has completed the last volume of his trilogy

[1] Der Autor hat den letzten Band seiner Trilogie fertiggestellt.

Declension

1 Singular Plural
Nominativ die Trilogie die Trilogien
Genitiv der Trilogie der Trilogien
Dativ der Trilogie den Trilogien
Akkusativ die Trilogie die Trilogien

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary