The First Night of Samhain

Find and Speak with Gunnar: First things first, you need to find Gunnar. He's chilling in western Glowecestre, specifically outside a house just northwest of the main stone bridge. Don't stress too much about the dialogue options when you chat with him; just pick whatever feels right. After your chat, stick with Gunnar and follow him through town. You'll end up meeting Tewdwr, and again, your response to him doesn't really change anything, so just go with the flow.

Escort Tewdwr: After that scene with Tewdwr, he'll want you to escort him north to a shrine for some prayers. Follow him, and once you get there, a short cutscene will play. This is rudely interrupted by three boars, so you'll need to take them down to protect Tewdwr. Once they're dealt with, chat with Tewdwr again. Now, it's time to head back south to find Gunnar at the house where you first met him in Glowecestre. Another scene will kick off, and you can choose your dialogue again. This is where things get interesting: you'll be dressed up as the Mari Lwyd, and your task is to go around Glowecestre knocking on doors to trick or treat.

Trick or Treat with Gunnar and Drinking Contest with Tewdwr: You'll find plenty of houses around where you met Gunnar. Just approach any door, knock, and pick your dialogue. You need to hit up three doors to fill up a progress bar. Once that's done, look for Gunnar down by the dock, south of the stone bridge. When you get close, a cutscene will start, and then you'll need to help Gunnar out in a fistfight against a guy called a Thickskull. He's not too tough, but watch out because his special moves come out faster than you might expect. After you've sorted out the Thickskull, go through the dialogue options. Your next stop is to meet Tewdwr again, this time in eastern Glowecestre, right outside the longhouse. Speak with him and agree to a drinking contest. This one has a few more rounds than usual, but honestly, it doesn't matter if you win or lose. After the contest, chat through the dialogue. If you choose the option with a heart next to it, you'll even get a brief romance scene with Tewdwr. Once that cutscene wraps up, Eivor will wake up, and that's the quest done!