A creature was trapped inside a lassimater for at least 320 years, mostly sleeping and dreaming of escape, only waking up when the Child of Music gently breathed into it. As the creature couldn't bear to live so long without the only person it cared about, so it conserved life energy to pass on to the Child of Music, allowing him to live as long as it did.
Help did come, in the end. After the Tenth Doctor attended the concert in its 320th year, he recognised that a creature was imprisoned inside the musical instrument. He later used his sonic screwdriver to free it. In its last moment, the creature sang a soaring, beautiful song that swelled with the glow of the light. The Doctor said this was the most beautiful music of all: the sound of freedom. (PROSE: Most Beautiful Music)