Dungeons and Dragons

I was looking to run a 5e campaign based around the movie The 13th Warrior. The movie, and therefore this part of the campaign, can be broken down into 3 sections:

1. The adventurers are called to a remote town that has been under attack. Houses furthest from the town center have been attacked and the families savagely killed. There's talk of an old evil that comes out when there's fog. They investigate the site of the most recent attack and find clues that may support the rumors. The next foggy night, the adventurers lay a trap and fend off a small band of attackers.
2. Once they recover from the attack, the adventurers are told to seek guidance from a nearby seer. The seer tells them to track the attackers back to their lair and kill their leaders, a "mother" and a "warlord." The adventurers find the lair in the nearby mountains, infiltrate, and kill the "mother." Unable to find the "warlord," they escape before they can be overrun and return to the village.
3. With the "mother" dead, the adventurers know the "warlord" will attack in force the next time the fog arrives. They spend the time they have preparing the village and villagers for the assault. The fog arrives, the "warlord" and his army lay siege to village. Once the "warlord" is killed, the remaining attackers flee with the fog.

The movie has a Norse setting and presents questions about the whether there is actual magic or just fear and superstition involved. In my campaign setting, I haven't made any thematic decisions yet. Obviously with this being D&D, the attackers can be magical but I think the more magic that's involved, the harder the campaign could be.

There are some important ways in which the campaign will differ from the movie.

First of all, there are 13 adventurers in The 13th Warrior (a surprise, I know 😏 ), though half of them are dead by #3. There will only be 3-5 party members.

Secondly, the characters in The 13th Warrior are all warriors (once again, shocking news 😏 ), i.e. martial classes, with no spellcasters among them. The party will likely have at least some spellcasters, which can make a big difference in tactics, especially if I keep the attackers caster-free.

Lastly, except for the protagonist of the movie, the warriors are clearly seasoned to some extent, which brings me to my question: Obviously the players will gain levels throughout this adventure, but what is the appropriate level to start this part of the campaign?