User blog comment:Nakano15/While on n terraria.../@comment-184.144.255.150-20130608235943/@comment-184.144.255.150-20130609050311

Here something a bit more detailed of what i meant earlier.

Once you reach level 100 a NPC( maybe the Guide ?) would give you a quest to go kill a special boss that would only appear once you accept the quest.

Once you kill that boss he would drop the loot you need to craft the rank stone.

It could be another kind of soul and then you you could craft all 7 kinds of souls to get the Rank Stone