Necron?

[Wolf Kanno]

Dr. Tot mentions "a great power sealed in the crystal" the murial on the wall in the summoners village relays the story as well...

...They aren't talking about Alexander?

The stories give the impression something "bad" was sealed. Though I doubt anyone would argue that Alexander isn't a "badass" after it's fight with Bahamut at least Alexander seemed like it can be controlled. Besides, Alexander was summoned with the help of only two shards of the crystal; Necron appears after the complete crystal is destroyed.

Also to Boko... hey I'm allowed to be a serious asshole at least three times a year

But to counter, if Necron is believed to be the antithesis of the Crystal which is life. Then Necron can be thought of as an avatar of death thus dying of any sorts means you'll probably meet him. I seriously see no real in game evidence to suggest he's a core to the Iifa Tree.