- #66
IIRC there was some foreshadowing of it in Avatar but it could be new.
Well anyways, at least DeS power-up receives a consequence.
The concept of "forbidden magic" being consequential was introduced during the Tenru Island Arc. Azuma died to his own magic and (forgot his name) Capricorn, or human takeover guy, died due to the consequences of taking over a spirit.
All the "7 kins of purgatory" were explained to utilize magic close to the source which was claimed to be darkness, aka the source that empowers Zeref.
As for Gray specifically there was alot of foreshadowing. It started with him gaining his tattoos when he tanked Mard Geer´s wtf attack, later during the Avatar Arc it was implied that his "disease" was eating at him, which he tried to battle through training, which didn´t work.
Wether or not Hiro implied that he left Juvia to protect her from himself is debateable, but during the arc there were panels dedicated to him noticing hat his tatoos grew larger and him getting worried over it.
During the war arc he had also another discussion with Laxus, where Gray mentioned Laxus cancer and Laxus Gray´s worsening condition. It wasn´t ever specified what exactly was happening, but worried looks at his arm and growing black demonic tattos could be cosidered foreshadowing.