Given the sudden consternation of combat, it would be very forgivable not to notice the horse that had come with Shoalar and the others suddenly rear in panic and bolt! Someone who knew horses, and who had time to think about it, might wonder why the horse would bolt INTO the magic darkness. But in the time it took to wonder that, it was already too late.
Aridha plunged into the supernatural gloom and immediately resumed her elf shape, but only for a moment. An instant later she was reshaping again...into something far worse than a horse, if almost as large.
A giant wolf spider had good enough eyes, but they didn't depend on them. Up close, they could feel vibrations in the air through incredibly sensitive hairs all over their body. It was an odd, alien sense...like being able to touch something without actually touching it. But it gave her everything she needed to strike at the man in the dark as if she could see.
(Aridha moves to R8 as a horse, unwildshapes, then wildshapes into a Giant Spider. Then she finishes her move to Q7 as a spider, since she can then detect the enemy there. [roll0] ([roll1] with advantage) to hit for [roll2] damage, and on a hit the victim must succeed a DC 11 Constitution save or take an additional [roll3] poison damage. Aridha gains advantage in melee against targets in the darkness who cannot see, due to a giant spider's 10' blindsight ability.)
Aridha's Current AC: 14 (foes that cannot see her are at disadvantage to hit her)
Form HP: 26