In my campaign, a character used witch bolt against an illusionist and hit, dealing 12 damage. The illusionist proceeded to cast mirror image.
Witch bolt states:
On a hit, the target takes 1d12 lightning damage, and on each of your turns for the duration, you can use your action to deal 1d12 lightning damage to the target automatically.
While mirror image states:
Each time a creature targets you with an attack during the spell's duration, roll a d20 to determine whether the attack instead targets one of your duplicates.
Assume the initial attack hits the actual target. Does the ongoing damage from witch bolt (after the initial hit) automatically hit the target, or does a roll occur to determine if it hits the right one?