Hello,

The card of the Shadow Rune Relic says that you recover one fatigue each time you kill a monster with this weapon. However, what happens, if you kill several monsters at once by using a blast weapon?

Nuwanda said:

The Shadow Rune is a weapon itself. The effect only applies when you are using the Shadow Rune to attack, and the Shadow Rune does not have Blast.

However, if you have found a way to make attacks made by the Shadow Rune weapon hit multiple figures (ie: Leoric's Heroic Feat), then I would say that the hero regains fatigue for each monster killed. That's mostly just a gut impulse, mind you.