User blog comment:FairyRaven/Fairy Tail Chapter 399: Wings of Despair, Review and Commentary/@comment-7418318-20140908160616

So this part is called "Ultimate Pain", it already has Acnologia in it and Igneel might also show up...

Could this ultimate pain be the death of Igneel?

Imagine it: After years and years of searching, Natsu is finally reunited with his lost father, only to lose him again. That would certainly qualify for the part title.