You need Second Sight (effect from drinking Dude's Juice) to see the true form of void creatures
To kill Magnar's void form you need the Ethereal Torch, a Godman weapon obtained from the Shard. You need 9 INT to get either Yngwar or the Ferryman to tell you where to get it. There are many ways to go about this quest, but you need to figure out the proper Rune to check, as well as where the Shard was thrown.
As far as I know you cannot kill the Restless Spirits. Just don't fuck with the graves and move quickly when hunting for the Shard
Side note: void creatures will hit you with Energy damage, whereas Restless Spirits strike with Electricity damage, similar to Bilocation shades... meaning the ghosts are closer to psionic projections than fully-infused void creatures (like Magnar)