First of all most of the things in this mod are faction exklusive for Solland/Ostland.
You can play as any other faction and Solland will appear as ai-controlled faction but there ar some things that are triggered by a script, which only takes place when Solland is controlled by a human player (you).
Currently there is no work-around or overwrite for this since I want the things to be exclusive!
(more detailed descriptions will follow after releasing the last big update for this mod)

Gerath von Ultingen
Gerath is your faction leader and a legendary lord.
There are no additional requierements for unlocking him. He should be available after confederating with Solland/Ostland

Darak-kal
Darak will spawn at campaign start in the south of the map.
He is a legendary lord and a "crusader" lord meaning he is a standart general and horde at the same time.
Darak will only spawn if Ostland/Solland is controlled by a human player.
That means he will be not available after confederation!

Patriarch Marius
Marius will be unlocked for recruitment after you build the "Cathedral of Pfeildorf" a unique landmark building in Pfeildorf.
After its completion he should spawn within your lord recruitment pool.
He also is a legendary lord.
As for Darak he is added via script that only works when Ostland is human controlled.

Seth of Ostland
Seth is a legendary hero.
He should spawn after spending a skill point in Gerath's first unique skill.

The buildings are only available for the faction of Solland.
Since the buildings are the requierment for the units, the units are exclusive too obviously.