Skiadrum

Skiadrum was a Dragon known as The Shadow Dragon, and the foster father of Rogue Cheney. He taught Rogue how to use the Magic once fabled to be able to kill Dragons themselves: Dragon Slayer Magic, giving Rogue the characteristics of Skiadrum himself.
However, Skiadrum fell ill and asked Rogue to kill him, thus ending his life.
(Source: Fairy Tail Wiki)