Artoria Caster sacrificed herself to the Forge of the Inner Sea to forge a new Holy Sword, which conferred the power of Holy Sword Creation on her; this power made her the ultimate threat to the continued existence of Fairy Britain, and all of the Threats to Humanity that lurked within, such as the Demonic Dog Barghest, the Remnant of Albion, the Calamity of Curses, or the Worm of the Abyss. Especially the last one, as the Holy Sword is both nourishment and poison to Vortigern.
After defeating the Calamity of Curses, Artoria Caster returned to Avalon, to sleep forever, and the conceptual "Guardian of the Holy Sword" was forged in her image. This Guardian is Artoria Avalon, who carries the same Anti-Threat to Humanity quality, as she exists to safeguard the world of humans.