Spellforce 3: Soul Harvest is the first stand-alone expansion (so to play this expansion you don't need to own the main game) for Spellforce 3 released in 2019. and already included in Spellforce 3: Reforced.

You're no longer playing as Tahar, the hero who stopped Rondar Lacaine from summoning Aonir. The story only mentions that one day the Crown was informed about many villagers dying without a trace. The Queen dispatched Tahar to investigate it. During the investigation, the dispatched group led by Tahar were ambushed by a mysterious group of Dark Elves, The Crescent, and the leader of the Crescent, the Masked Woman killed Tahar.
You're now playing as General Aerev (similar to the main game, you can choose both the gender and appearance of the main character, but only two out of nine skill trees can be chosen, while in the main game, you could choose three of them) who is living in disgrace after being responsible for the death of their company many years ago before the event of Soul Harvest. The Queen grants Aerev a second chance, making him/her the commander of the Wolf Guard and orders him/her to investigate several incidents that seem to indicate that the God of Light is again trying to enter the world once more.
The game is similar to the main game (and I'm not going to describe the game mechanics once more since probably all players will play the main game before any of the expansions, even though they are stand-alone), but there are some changes. The first one which you'll notice is that you can create mercenaries who can replace companions. While going through the story of Soul Harvest you will not have Elves and Orcs at the Wolf Guard's disposal since they're assigned to other quests. They are replaced with Dwarves (favor defence) and Dark Elves (favor offence).
Why this expansion is not as good as the main game is mostly because the story isn't as intriguing and is filled with side quests that require you to constantly go back to locations you've already been at the beginning of the Soul Harvest story. Even though many of the side quests/bounties are repetitive, they at least offer really good gear in return for your time.
What also I was annoyed with was the final boss fight. Before saying something, I have to point out that the last battle was really good, you got the chance to fight with your ally against the Orcs and the Crescent in a medium-scaled skirmish battle. This time, unlike I pointed out in my review of Spellforce 3 "Uneasy Alliance" quest, here your allies are much more useful and they try to attack at the same time as you to make your enemies harder to defend.
The final boss fight is, unfortunately, nothing outstanding, not even close to that, you're not fighting against the boss, but against dozens of waves of enemies from different races that the God of Light creates. Then you have to generate a small army with a small amount of resources to defeat three waves of enemies on the other side of the bridge and at last, fight projections of bosses you have already defeated before and projections of you and your active companions that deal three times less damage than you, but have ten times more health. The "last boss fight" took too much time to complete and was too annoying to have any satisfaction at the end.

All in all, a smaller stand-alone campaign that offers two new races and the ability to have mercenaries instead of companions you meet along your journey. The unsatisfying final boss fight unfortunately left a sour taste in my mouth at the end. To sum up, it isn't as good as the main game, but overall this is a good expansion for Spellforce 3.