The legend of Hasi, the spirit of negativity. He is the rival of Chanya, the spirit of positivity. He can possess others and take all the positivity out of them, but it only works if they're feeling negative emotions, such as doubtful, anger, hatred, or sadness. He sensed a negative emotion in the Pride lands. That was Kosa and Mwerevu arguing about Nzuri. Then when all the Guard left.. Hasi chose Kosa as his host.
Hasi got inside Kosa's mind
Hasi: Kosa, Leader of the Lion Guard, Protector of the Pride Lands.
Kosa: who are you?!
Hasi: I am Hasi, the spirit of negativity. I have come for you.
Kosa: Wh-Why!?
Hasi: For you to be my host.
Kosa: wh...what? Never!
Hasi: I'm afraid you don't have a choice. Muahahah haha hahamuahahah !!!