Fatal in Portuguese, much like in English, means 'deadly' or 'fateful'. It's a strong word that immediately grabs attention due to its dramatic connotation.
In the context of the song, the singer laments, "Minhas noites são fatais" (My nights are fatal). This isn't about literal death, but rather a metaphorical 'death' of spirit or hope, implying that his nights are overwhelmingly bleak and destructive to his emotional well-being. This evocative use of the word makes it particularly interesting.