In her last appearance, though, she's replaced the drill with some kind of... hook... thingy. So clearly that can't be it.
If it requires an adult phoenix to transform a juvenile into an adult, why then are the royal siblings being made to kill each other? Wouldn't it simply make more sense for the royals to choose the most deserving and age them, rather than pick the sibling who proves Ax-Crazy and ruthless enough to murder all their own siblings?
It is implied that if the survivor doesn't have the qualities of royalty, they just pass them over and start the war of succession all over again with the next generation.
In the first chapter, Hime told Flandre that it's forbidden to make a Blood Warrior out of someone you've killed yourself when she refused to revive Wildman. Later on, the fact that Severin killed Mikasa before reviving her as his Blood Warrior is something he likes to taunt her about. Is this meant to show that Severin is a monster who doesn't care about the rules, or is it just a retcon? And if it is the former, who makes these rules and how can Severin get away with breaking them so casually?
It's there to prove just what a monster Severin was. This is also the second taboo he has broken - the first one being the forbidden use of zombies.