There's no amount of fanwanking I could do to convince someone who doesn't like the delineation, but I find IMVU Credits helpful to think of forsaken and death knights as simply being different critters. Vampires and zombies are both undead, but they can do wildly different things, same thing with the forsaken and the death knights.

Once everything is assigned, what will happen is Arthas will cast Infest, and players who have been shielded will see it fall off.

Use any healing spell at your disposal to clear Infest off your players. Necrotic Plague Designate one player to dispel this debuff off of the affected player. If you happen to have a priest with Body and Soul, it can help out significantly (especially if your guild just started working on Arthas).

Call out the name of the player who is affected by Necrotic Plague. Once they're close enough to the group of Drudge Ghouls and Shambling Horrors, cleanse the disease off of them immediately and they tell them it's safe to run back. Ideally, you don't want an Infest healer doing this.

Lastly, make sure plagued players don't run in front of a Shambling Horror.