The two aligned planes - Elysium, the sanctuary of Good, and Stygia, the bastion of Evil - are governed by jealous Powers who actively attempt to drive out all characters who are not native to them. Any character visiting these planes who is not native to the plane – including Mortals – will take 3 points of planar damage whenever they move around (with the exception of entering/exiting buildings). This applies whether the character is moving around outside or inside, and no armor will protect against such damage. Only Demons (a term which includes Fallen Angels) with -20 Morality or less are native to Stygia, while only Angels (a term which includes Redeemed Demons) with 20 Morality or more are native to in Elysium.
This damage can be negated entirely by either learning the skill Planar Protection or by drinking a Potion of Planar Protection, though the latter method works only for a short time. Certain Faction Upgrades (Planar Screen, Planar Domain) can provide a limited form of planar protection.