As seen in "The Skyrim ABC Challenge"(https://youtu.be/vLa6Qh7JzeU)

Instructions:
Install using a mod manager. I used Mod Organizer
Enable in-game using the "ABC Kill Challenge Control" power you get upon first loading into the game.
Alternatively you can use "startquest 6T4ABCKillChallenge" in the console but it wont show the quest is active until you kill an NPC that starts with the letter A

Notes:
- Requires SKSE
- Tested and creating on slightly modded version of Skryim AE, (thought it should work without AE) I used the ADT modlist as a base - https://github.com/Styyx1/ADT
- Should work with all modded NPCs as well, but I didn't test on any of them.
- The way this mod works is a little weird, if you long distance kill an NPC without coming close to them first, there's a small chance they won't count towards the quest. You have to get close enough to the NPC so it gets tracked first, and then you can long distance kill them.
- Kills from a follower count as kills so be wary of that lol