Warhammer Age of Sigmar: Storm Ground

Stormcast

Heroes:

Freya Skyhelm | Lord Celestant - Hero
Alrik Nightrunner | Lord Aquilor - Alternate Hero

• Shows up as a mission in the easy campaign which will have an Ally which you can control, you need to keep this unit alive. You'll then get given a couple specific follow up missions related to that unit. These will be their own mission node separate from the usual 2 mission choices. Once you've completed this questline and successfully completed the campaign, you then unlock this unit as a permanent Hero choice.

Units:

Castigators
Liberators
Prosecutor
Vanguard Hunters

Knight Incantor - Elite
Retributors - Elite
Vanguard Pallador - Elite

Check the mission rewards when choosing which of the two missions to select to see if you get a unit as a reward. I also went out my way to get most chests within missions, however can't remember if these can give units too.