Killed NPC by accident (?)

So, i was in the forbidden woods exploring, when i found what seemed to be an enemy pretending to be dead, i dashed toward it and attacked but it got up saying something like "have you lost your mind" or something and transformed into a powerful beast with lightning powers, i was barely able to defeat it, but then it died saying we're all beasts, was this an important npc or just a secret miniboss? Did i just screw up?