++++++++SPOILERS+++++++++++++++++++++ They didn't disappear, per say, they became apart of the dragon slayers, in order for them to develop anti bodies that prevented them from transforming into a dragon like Acnologia.
posted एक साल से अधिक पुराना
looks like your up to तारीख, दिनांक on the मांगा as well!
Well those weren't fused with the Dragon Slayers I believe some of them were slain द्वारा Acnologia and some left maybe are hiding from him या something worse.
