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Tear Jerker: Wakfu
The end of Episode 19. Full stop.
The death of the Soft Oak in episode 13 is pretty sad too.
And of course, Sadlygrove's death in episode 25.
Sadlygrove: I'm not dying, I'm entering the legend.
Nox has spent 200 years of research, plotting, and genocide in order to gather the energy needed to travel back in time and stop his family from being destroyed partially due to his own negligence. In the end, he manages to defeat the heroes, drain the Tree of Life (killing the Sadidas in the process), and uses the all the energy gained over this time period to travel back in time... a whole twenty minutes. The ending would have been happier if he had completely won.