Headlamp extra batteries, extra flashlights, rain gear, food, water, fire starter. They say bears are more active at night so watch out for those guys.
My little 2 cell AAA mini-mag is always in my pocket when I'm out. Been caught after dark too many times not to carry it. I prefer the little mag over a headlamp because it shines much further and lets me look for eyeballs side to side and way ahead.
Hiking after dark is not a good thing if it can be avoided, but I have had several experiences where it became expedient to pack up and get out quick, requiring several hours of travel after dark.
Depending on the location I have done a lot of night hiking. Won't do it at place like Red River Gorge as the cliffs and drops into the rivers often have Mountain Laurel growing where you cant see the edges. However place that don't have that kind of terrain I hike often at night and enjoy it. Typical if I do this I am planning on spending the night out so I take y back, sleeping bag, sleeping pad, and all that stuff. I really do not carry anything different except for my headlamp is on my head and and not in the pack or a pocket.
One thing I really like about night hiking is that the hills are easier. Since you can't see how much further you have to go to get to the top you don't worry about it lol. Only thing that ever made me jump at night was an owl that scarred the crap out of me. Walking down the trail and it must have been going after something to eat. Swooped out of a tree and right across my path about face high and was so close I could feel the breeze it created. Was actually kind of cool after my heart stopped racing and I realized what it was.
I don't night hike anymore. However I do carry a good light with me everywhere I go out just in case I am caught in the dark. It depends where you are, but if you are in one of those states that is hot in the day and cold at night, you might want to make sure to have warm gear to dress in.
First aid kit (always the first thing in my pack).
Quality headlamp with extra batteries.
Small LED backup light on a chain around my neck.
Appropriate clothing and footwear.