Grand Magister Jurek:
You must slay either Baium, Jaken, Anakim or Lilith so that additional souls can be absorbed into the level 10 Soul Crystal. If you do so, it can become level 11 (a level 11 Soul Crystal can become level 12). Their souls are so powerful that it is said that the Soul Crystals of a whole adventuring party can absorb the soul.
Level 11 and 12 Soul Crystals can be obtained from Raid bosses like Beast Lord Behemoth, Roaring Skylancer, Minas Arnor, Eilhalder Von Hellman, Krokian-Padisha Sobekk, Priest Chloe of Antharas, Siren and Tyrannosaurus. But it is not easy to absorb their souls compared to Baium, Jaken, Anakim and Lilith, and their souls are weaker -- only one member of the party that defeated them can absorb the soul.
Although you can get level 12 Soul Crystals that way, it is much more difficult to obtain a level 13 Soul Crystal...
Keep listening.