"So, this is the seventh form of Darkness Eye!"
Merlin's consciousness was battered by an endless onslaught of negative emotions. An average Spell Caster or even a Spell Caster who had consolidated a Mind Heart, would not be able to withstand it.
Only those Spell Casters whose consciousness had been taken control, and became puppets would be able to withstand the onslaught of negative emotions, hence successfully cultivating the seventh form of Darkness Eye.
It was not surprising that despite the Legend of Darkness' unwillingness, he still handed over the cultivation method of Darkness Eye, and did not try to interfere. That was because the final step of cultivating the seventh form of Darkness Eye was already inherently filled with unimaginable danger, almost to the point of death.