Master Guardians are the powerful creatures within the Arcana monoliths; they will only respond to one who can harness their powers. You, as the next destined Lord Ruler, are tasked with gathering the Arcana and ruling over them.
Unlocking the ArcanaEdit
Upon reaching Horodyn, you are stripped of your abilities and memories; you will have to start from the bottom up, re-learning everything. As destiny would have it, you join a guild and eventually face the embodiments of the Arcana: the Master Guardians.
In due course, you face them; ultimately, they submit to your will as the next Lord Ruler.
Harnessing the ArcanaEdit
Taking the Arcana's power for yourself is straightforward : locate the Master Guardian, and defeat it. In so doing, you both subjugate the guardian and acquire their 'lairs', opening up more missions and more opportunities to gather monster cores, thus fulfilling what was your destiny : to Rule over All the Arcana.