++++++++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 zaidi ya mwaka mmoja uliopita
looks like your up to tarehe on the manga as well!
Well those weren't fused with the Dragon Slayers I believe some of them were slain kwa Acnologia and some left maybe are hiding from him au something worse.
