Brock's Steelix evolved while Brock was away.
During the battle at the Hanada Gym against Kiawe's Turtonator
revealed that he was wearing a Steelixite Mega Stone necklace which he triggered so that Steelix could be fully powered to fight off the fully powered Z-Moves he heard about. Mega Steelix endured the attack but it had quite the effect on it, just as Brock
expected it to but it wasn't enough to knock it out. Mega Steelix quickly attacked with a Stone Edge to knock out Kiawe's Turtonator
and win the battle.