Weisslogia was a Dragon known as The White Dragon, and the foster father of Sting Eucliffe. He taught Sting how to use the Magic once fabled to be able to kill Dragons themselves: Dragon Slayer Magic. White Dragon Slayer Magic gave Sting the characteristics of Weisslogia himself. However, Weisslogia instructed Sting to kill him and bathe in his blood so that he could become a "true" Dragon Slayer.

(Source: Fairy Tail Wiki)