S'emparer means 'to seize' or 'to take hold of'. It's a more active and impactful verb than simply 'to take', suggesting a forceful or complete takeover.
In the song, the lyric "Et l'ennui s'emparer du ciel" (And boredom seized the sky) uses this word to personify boredom, giving it the power to completely engulf and dominate the atmosphere, reflecting the heavy, melancholic mood of the song.
Marc Dupré's "Là Dans Ma Tête" is a really relatable song about a breakup that's hard to get over. He sings about seeing the relationship fall apart, with lines like "J'ai vu tes yeux pleurer plus d'une fois" [I saw your eyes cry more than once] and "Nos mains se détachaient peu à peu" [Our hands detached little by little]. It sounds like they both knew it was time to end things, even saying "Continuer était inutile" [Continuing was useless].
But even though they chose "la liberté" [freedom], the person is still stuck in his mind. He tries to move on, wishing he could "effacer de ma mémoire" [erase from my memory] and "Oublier ton nom et ton départ" [Forget your name and your leaving]. Despite all that effort, the chorus repeats, "Pourtant, tu es toujours là dans ma tête" [Yet, you are always there in my head], showing just how difficult it is to truly let go of someone you cared about, even when the relationship is over.