well. I think the thematic equivalent would be something like firestorm. which technically the player does not have to target, but yet it doesnt effect immaterial. I think this would be similar. the prowler would have to be able to touch something to attack it. immaterials are thematically untouchable. so i guess no, it would not affect immaterial creatures. Course, paired up with a thorn shield, they would still die, albeit slower.