Adds a whole new faction which is extremely fun to play. The viking and Scandinavian theme is interesting and the art and troops reflect this beautifully. The new troops and wielders have their own unique and fun play styles. For example, there is a new crow-man troop called the Korphan which can leap the same distance as their movement points. This leap allows them to maneuver around obstacles and zones of controls and thus gives a more viability to melee builds to combat ranged and magic builds. Plus, it's f**king epic to leap into a troop of enemies and just demolish them. Some other troops have similar unique abilities such as the Norror which can teleport to an enemy to melee attack them and then retreat instantly.
Wielders also have layouts but I haven't played them enough to test them out. Though there's one wielder with chaos essence and destruction essence generation from the start.
Someone gave a negative review complaining that the DLC does not have a campaign. This is true. I believe the developers were working on the Vanir along with another faction, Roots, which was going to be in one bigger DLC. However, to get the content out faster they decided to release them separately. But, the stories of the Vanir and Roots are intertwined and thus the campaign was pushed off until the release of the Roots and possibly later. The developers should explicitly state that there is no campaign currently in this DLC, but no where in the advertising or store page is a campaign mentioned; concerns of false advertising are misplaced. Further, the DLC is still worth it's price and more. Songs of Conquest is an incredibly high quality game and the Vanir faction is no exception. New art, music, and gameplay is incredible to experience. The developers worked hard on this DLC and deserve more than they charge. Even without the campaign there is still plenty of content to play with the Vanir.
If you have liked Songs of Conquest and want new play styles and diversity, the Vanir DLC is definitely a buy. The price is cheap relative to its quality.
10/10
I might revisit this rating and review upon further reflection of balancing.