Complex sentence: Nimue would later be stabbed by the dagger, allowing her magic and consciousness to possess her attacker; thus anyone who killed a Dark One gained magic and the Darkness grew more powerful with each new Dark One.
Split into simpler sentences: Nimue would later be stabbed by the dagger, allowing her magic and consciousness to possess her attacker. Thus anyone who killed a Dark One gained magic and the Darkness grew more powerful with each new Dark One.