The word anochecía is the imperfect form of anochecer in the first person singular. There are other translations for this conjugation.
See the full anochecer conjugation

anochecer

Ya anochecía cuando terminó su obra.
When he finished, it was already getting dark.
Prendieron a Pedro y a Juan y, como ya anochecía, los metieron en la cárcel hasta el día siguiente.
They seized Peter and John, and because it was evening, they put them in jail until the next day.
Como se anota arriba, cuando Jacob escogió donde acampar, era simplemente porque anochecía y ahí es donde se encontraba en ese momento.
As noted above, when Jacob chose this place to camp, it simply because night was falling and this is where he happened to be.
Ya anochecía cuando Cuauhtlatoatzin regresaba, a casa, quedándole de camino el cerrito del Tepeyac, y cuando pasaba cerca del lugar, escucho una voz que lo llamaba de lo alto del lugar, y decidió acudir.
Then he returned at the end of day, then came straight from there to the Tepeyac Hill, and when passing near the place, he hear a voice that called him to the top of the site, and decided to go.
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