PAD's All-New, All-Different X-Factor: From The Beginning
X-Poster of the Month & Year: (August) 2012 | March 2013
Come to Deathstrike & .A˛: Mutant League Champions 2013
Well, as I understood it, the Phoenix is the life force (I am life and fire incarnated!). When it bonds with someone this host uses the life of the universe, the energy of the Phoenix. That is why the Phoenix broke her bond with Rachel, to avoid her aborting future life. And I think that is why Jean says "They will not let me stay here for long" when she came back at the end of Morrison's run. That is a nice detail about Phoenixes: They do not get to live long while keeping the power.
Of course, the actual event shows the Phoenix feasting on a planet for nourishment when it still lacked a host, which is not really coherent with that explanation.
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