*Slash*
Gideon finishes off a mutated gold-grade Galarian and sat on the ground while absorbing its soul orb.
Rank-3 to Rank-4, Rank-6 to Rank-7, and Rank-9 to Rank-10; these three advancements are special for an Arcana Master.
When one reaches peak stage at such soul ranks, they would need to kill either a similar attribute beast of the same level or any other beast of a higher rank to advance to the next rank.
Usually, the former method is more preferred as it would raise the ability of one's Arcana. However, the most sought out is still the one with both of them, like a rank higher with the same attribute.
At a lower advancement like rank-3 to rank-4, finding such a compatible beast is pretty much doable for any person as most of the Arcana Masters will be in their schools. They have a lot of seniors and teachers to guide them.