Shadowspawn are undead beings that spawn from dark blood.
Corpses animated by dark magic, with only one order in their minds. Destroy.
Contrary to their in-game description, Shadowspawn do not have any armour and have a speed of 'fast' instead of 'slow'. They can be considered as improved zombies, trading slightly lower numbers for more health and speed as well as being able to be spawned from fallen barracks units by most other enemies introduced in the Forgotten Treasures mini-campaign.
They come in groups typically, so druids and barracks are one of the best ways to go; either Arch-Druid Henge for damage spreading and extra blocking ability with runed bears, or weirdwoods for extra damage and occasional stunning will work. Blade Dance will also deal good damage to them as will Eerie Gardener. Eldritch Doom will also be useful.