10 of the legendary heroes are part of a faction at the start of the campaign. 2 other unknown characters are obtained by Aesling and Bjornling per the mod description, though the method to obtain them is unknown since I haven't played their campaign yet. The final Legendary Hero, which faction they're part of, and how to obtain them is all unknown.

If anyone knows the names and unlock conditions of those last 3 characters, please comment with the info.

Lokjar Bjoergir - Starting hero of Dolgan
Soren the Hoarder - Starting hero of Skeggi
Rafn Oflatisson - Starting hero of Baersonling Tribe
Kolsveinn of Skraevold - Starting hero of Aesling
Verdandi Sea-Maiden - Starting hero of Graeling
Ulsdau - Starting hero of Tribe of the Wolf
Graill - Starting hero of Tribe of the Wolf
Zar Uzelek - Starting hero of Yusak
Oda Winterclaw - Starting hero of Bjornling
Oerl the Young - Starting hero of Ursfjordings
unknown - obtained through unknown methods by Aesling
unknown - obtained through unknown methods by Bjornling
unknown - obtained through unknown methods by unknown faction

These 4 factions do not start with a Legendary Hero: Kul, Sarl, Icefang Tribe, Varg