The Formation of the Darkness, the loss of Wario's control over his anger, and the destruction of Merlin's Brother may have all come from one single event, but it is time that we dealt with each in turn. As Mario and his friends found out this season, Wario would do anything to finally destroy the Mario Brothers, tricking the group into recharging the Nether Saber and using it to go after them.
That is, if they had to go after him themselves. With a returning WaLuigi, The Brothers and their trainer came before Wario, and fought together to finally defeat and cure Wario of his rampaging anger, healing Wario's soul and putting the first step towards the Two Chosen Ones and each of their destinies in the future.
Wario and Mario's destinies are intertwined in this battle, what will be this battle?
And what about the Darkness? Who knows.
All we can do now is rest and regroup.