To start the main quest, you first to need to find a dynamite that have 3 locations.
At spawn before jumping into the Pack-A-Punch
Behind Juggernog
Left to the power room
After that, you'll need to place the TNT on a safe, behind the counter of the garage. You can now get the key to the chest at the back of the power room, which contains the Wunderwaffe DG-Scharfschütze.
You now need to shoot 3 light around the map with the Wunderwaffe. You can shoot all 3 of them from the main area, but it will be way easier if you have crafted the trample steam.
Now the Relic will spawn at the center of the map, wait till the Relic gets colder, then grab it and place it on the cooker at the Diner.
Now you can cook the Relic by interacting with it but be careful, as soon as you'll press F, a lockdown will start.
And this lockdown is pretty hard, so be sure to craft both the trample steam and the shield, and to have the Wraith Fires.
Place the trample steam to one side of the bar to make it easier, especially in solo.
Disclaimer : After this step Pack-A-Punch will be disabled till the end of the quest. So be sure to upgrade your weapons BEFORE doing this step.
You need to kill an electric crawlers in front of 3 generators around the map.
At the green tent, at the back of the map
To the left of Juggernog
At the Double Tap area
Now grab the Relic at the cooker, this step is timed and you'll have to pass near some bins to charge back the Relic.
Be sure to have a trample steam, ready to throw you into the Pack-A-Punch area.
A beautiful map to show the path to follow
Now interact with the Pack-A-Punch to start the final step.
First, the Pack-A-Punch will transform into a giant soul box.
When it's been fed, the Relic will start floating, and you'll have to follow it and stays inside its dome. At some point the Relic will stop and you'll have to feed it with soul to make it move. You'll get a max ammo bonus after every stops.
This step is really hard, so I recommend you to get solid weapons such as the Ray Gun MKII (the original Ray Gun is not that good considering the tight space you'll be in this entire step), the Thundergun and the Monkeys Bombs.
The tight area you'll have to stay in.
After completing this step, congrats you've done with the main quest, you can now enjoy the map in nighttime and get access to the free Ray Gun MKII side Easter egg.