I have finished watching The Shimmering Shells episode.
In the first half of the episode, Stella, Bloom and Aisha must confront their doubts about themselves in the Labyrinth of the Abysses to find a clue to the Gem of Self-Confidence. The Winx are entrapped in an illusion that makes them confront the people that are the source of their doubts. In particular, Aisha confronts an apparition of Nabu.
In the last season, Nabu had sacrificed himself to close the Black Circle's abyss in Tir Nan Og. The Winx had tried to use the Gift of Darkness to revive Nabu but Ogron wasted it on a flower leaving Nabu doomed to whatever fate was in store for him. What had actually happened to him in that episode was obviously ambiguous. In the original version, Morgana mention she and the Earth fairies would take care of Nabu until he woke up for Aisha, but the way Sky placed Nabu's staff on his chest made it seem as if Nabu was a fallen warrior--as if he had died in battle. Wrapping up his body in flowers did not help the image either. However, the fact remains that Morgana said that she would take care of Nabu until he woke up, meaning that Nabu was simply in a deep sleep, maybe a coma or an enchanted slumber.
At first, I thought that this had been a case of Never Say "Die" or Disney Death. Either the writers had actually let Nabu die but due to various reasons did not want to use the word "death" and its related terms or Nabu really was in some sort of slumber and were planning to use him in the next season as a plot device.
Now, if we look to the Shimmering Shells episode, Aisha confronts an apparition of Nabu that accuses her of "letting [him] die."
Aisha: Nabu, I miss you so much!Those are some harsh accusations! Granted, this is not the real Nabu speaking, but an illusion created by the Shimmering Shells and possibly Aisha's subconsciousness. As the audience, we must ask whether this is an accurate display of the reality of those events. Does Aisha see Nabu's actions as the cause of his death? Is he really (biologically) dead? Or does Aisha just view his unconsciousness as not living his daily life?
Nabu: So why did you let me die? You let me die! You could have saved me, but you let me die.
After so much time, we still do not have an accurate answer!
Season four's latter half definitely gave the series a darker tone after the introduction of the Earth Fairies and their need for revenge.It hit its darkest moment when Ogron wasted the Black Gift and let Nabu fall into his (pseudo?) death. I believe that this helps it appeal to its older audience, but as usual, the series abounds with story inconsistencies.
Personally, I think Rainbow and Nickelodeon have invested too much into Nabu's "death" to just leave him dead. Dafne apparently does not have a physical body yet she is a spirit bound to a lake. Isn't the fact that she has no body considered "death" too? With the advent of Dafne's role in season five, I think they might use her situation to somehow bring back Nabu.