Synonyms that are in the dictionary are marked in green. Synonyms that are not in the dictionary are marked in red.
Antonyms that are in the dictionary are marked in green. Antonyms that are not in the dictionary are marked in red.
While the game normally doesn't explicitly name the enemies, Nihilanth gets its name revealed as the title of the final chapter.
Source: https://gamerant.com/half-life-game-how-beat-nihilanth-final-boss/