Ghostwings are a very mobile shadow species who are highly reliant on Mana.
Ghostwings are a case where their utility is almost entirely dependent on their Skills. With the exception of their base Strength, their Stats are rather poor and could use a boost across the board. Note that their Health Growth is very low, while their Speed Growth is quite good.
To make full use of a Ghostwing it's recommended to have a full shadow team that can create lots of shadow tiles. Their two fixed skills, LifeTap and ShadowFlight, both create shadow for their ShadowArmor onTurnEnd and ShadowBound trait. If they get ShadowStep that further boosts their mobility, but that also requires shadow. Given the right support, a Ghostwing can deal a surprising amount of damage with Swipe and BackStab in their potential skill set.
Alternatively, with the right equipment a Ghostwing can fully be repurposed as a support unit. Another possible skill they can get is Weaken, a rare debuff skill that combos well with Curse. They can also learn ShadowBond for tactical positioning, by using their Speed to advance forward then switching with a slower unit such as a Void Bringer, throwing the tanker in the front while retreating to a safe place.
Consider playing around with their Level Types. For aggressive use a Strength Level Type is great. They'll lose a little Speed, but gain more Health and Strength. The easiest way is via fusion with a Flayer, who can also get them the powerful Suffocate in the process.
Since they fly you can also upsize for more power without interfering with Overworld movement.
By default Ghostwings have no special use on the Overworld.