Warning: Possible spoilers ahead.
Rosenkreuz's true power runs on The Power of FriendshipJust as his reincarnation Spiritia shows us, she has full trust towards her close companions and vice versa, and that's what gives her their powers. It was also said that Rosenkreuz once had 8 close friends, making this a case of Generation Xerox. Though it may be obvious when you think about it, this WMG does fuel the one below...
If Iris ever get her hands on the "Blade of Rosenkreuz", she'll end up unable to use itBecause she lacked the friendship capacity that Tia has. Worse, it may even backfired spectacularly (This Troper reminisces how, in Devil May Cry 3, Arkham got all the ingredients to harness the power of the Dark Knight Sparda, but ended up turning into a blob, and his lack of compassion that Sparda had is the reason why that happens).
The Blade of Rosenkreuz is a real weaponJust like Grollscwhert or Weissilber, it is a sword that, going by its theme, store and harness the power of his close friends. When he sacrificed himself for the Empire, they're buried separately - it may explain how there are 2 walking reincarnations of him, with separate abilities.