The eternal template creates a creature that, for the most part, cannot be killed. It cannot even be killed for a while, the way ghosts and liches can. It is, in a word, eternal. The CR adjustment listed is an approximation of what additional resources a group is likely to use up when dealing with a foe that has little regard for its own well-being, and is likely to return at least once before characters realize what they are dealing with and go to plan B.
Unlike most of the options presented in this product, the eternal template should not be used just to increase the CR of a foe to make it appropriate for an adventure of a higher level. Instead, this template is presented as a tool to facilitate certain kinds of storylines in ongoing campaigns. To reinforce how campaign-specific the use of this template should be, no sample monster is presented. Instead, the following advice on how to employ eternal creatures is offered as an extended guideline for GMs.
If a GM needs a foe that is nearly immortal, he can use the eternal template, coupled with a specific weakness that overcomes it. Such a weakness might be a special material (oak wood, jade, onyx, silk, or gold all make fine choices), essentially turning the entire template into a form of super damage reduction. Or the weakness might be conceptual (only a foe that knows the creature’s secret true name can slay it, or it can be killed only on a holy day of the religion it once worshiped). Mythology is full of creatures with such weaknesses, and the eternal template is a perfect way to represent some of the old “indestructible” monsters of yore. In any case, a Knowledge check that exceeds the normal monster lore DC by 10, or one coupled with research in a major library, should reveal an eternal creature’s weakness.
Eternal creatures that really can’t be killed should be limited in other ways. An eternal giant might be the guardian of a holy valley and be unable to leave it. An eternal dragon might sleep for all but one day every year. An eternal goblin might lack the personal power to be more than an annoyance. The GM should think about how the players are expected to overcome the eternal creature (if it is a foe), and why it hasn’t taken over the world.
Of course being indestructible is a far cry from being immune to defeat. Unless it can teleport, an eternal creature can be grappled, buried alive, thrown into an indestructible prison, or banished to another plane of reality. Requiring players to use one of these methods to overcome an eternal foe is a particularly harsh encounter, but it might be reasonable for a well-equipped, savvy group of characters.
An eternal creature cannot be permanently killed or destroyed. If killed, turned into an undead, polymorphed, petrified, burned, or otherwise destroyed, it returns to life and normal well-being 2d6 minutes later (even if disintegrated), gaining benefits similar to a true resurrection coupled with a heal. If the location of its body has become hazardous enough to kill it instantly, when the immortal creature returns to life it teleports to the closest safe space. Eternal creatures do not age, eat, sleep, or need to breathe, and they are immune to magic versions of those effects.
The downside of being immortal is an inability to grow and change. The eternal creature never gains any levels or Hit Dice. The effects of being eternal cannot be overcome with even a wish spell. Unless the eternal creature has a special weakness (see above), only the direct intervention of a deity can overcome the eternal status.