What CR adjustment should I make for Divine Rank 0

Question

How do I calculate the the CR adjustment when adding Divine Rank 0?

Note: Yes, I know that not everyone agrees with the CR but it is a useful overall guide. So, with that being said I need to make sure I don't completely murder my PCs.

Related: This Question is similar but it's asking a completely different question.

Context: I will have a 20-HD enemy (main boss) that will be granted divine status at Rank 0 while he is doing his bidding. (with the promise of higher status once complete.) I would like to make sure I place the encounter that is atleast possible to win.
(I am planning to have the party be lvl 18)

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Divine Rank 0 gives some, but not all of a god's benefits.

A quick rundown of quasi-deity abilities

There are many more abilities that they do not get until rank 1 which I didn't list since they don't apply.

Hit Points:
Deities receive maximum hit points for each Hit Die.

Speed
Deities can move much more quickly than mortals. (Medium biped: 60', 30 climb, 120 Fly if capable)

Synergy Bonuses
For every 20 extra ranks a deity has in a skill, the deity’s synergy bonus from the skill (if any) increases by +2.

Immunities Deities have the following immunities. Individual deities may have more immunities. Unless otherwise indicated, these immunities do not apply if the attacker is a deity of equal or higher rank.

• Transmutation A deity is immune to polymorphing, petrification, or any other attack that alters its form. Any shape-altering powers the deity might have work normally on itself.

• Drain: Energy Drain, Ability Drain, Ability Damage A deity is not subject to energy drain, ability drain, or ability damage.

• Mind-Affecting Effects A deity is immune to mind-affecting effects (charms, compulsions, phantasms, patterns, and morale effects).

Damage Reduction: 10/epic

Fire resistance: 5 + divine rank (0)= *Fire Resist 5

Spell resistance: 32 + divine rank (0)=SR 32

Immortality: All deities (even those of rank 0) are naturally immortal and cannot die from natural causes. Deities do not age, and they do not need to eat, sleep, or breathe. The only way for a deity to die is through special circumstances, usually by being slain in magical or physical combat.

Answer 1

This is from Deities & Demi-Gods page 56

Deities do not have challenge ratings.Entities of this sort are so far above the realm of mortal heroes that determining their level of power in re Relation to mortals becomes almost meaningless.

After that it also goes on to say...

If you feel the need to calculate a Challange Rating for a deity,try adding its total Hit Dice to its divine rank. The result may or may not be an accurate estimate of what level of characters could Challange this god...

Also for your information a divine rank of 0 is quasi- deity or hero-deities. A "Demi-God" is divine ranks 1-5 this information is found on page 25 of Deities and Demigods.

So with that said your "Quasi-Deity's" CR would equal the amount of hit dice he or she would have. So if your Quasi-Deity had 20 outsider hit dice as the norm of any deity you would add 20 to the number of class hit dice he would have easily putting him out of reach with a level 18 party.

But as you stated in your question your creature only has 20 hit dice so his "CR" would be 20. No adjustment for the divine rank of 0.

Answer 2

The monster manual has rules for creating monsters starting on page 296, I would recommend reading the entire section to get a feel for what the CR of a creature should be based on its abilities, ability scores, etc.

Drawing from those rules, in a very general way: CR is about 1.5 times it attack modifier plus 2, CR is roughly 13 plus total AC, Monsters with SR should have about 11 higher than their CR, A monsters good saving throws should be about 1.5 times their CR, its poor saving throws should be about equal to their CR.

"The two basic tools for determining CR are comparing and playtesting." page 302 MM Compare him to other creatures with similar abilities to those he gains from the deity level 0. Does he have more or less hit dice/ hit points, is his AC higher or lower, is he attack higher or lower, does he deal more or less damage, does he have more or fewer deadly special attacks, etc. If he is clearly superior to monsters of a particular CR he is a higher CR if he is clearly inferior he is a lower CR.

Play testing is more time consuming but but will give you a clearer picture. Create a team of char and play out the fight. If they start fresh, all hit points, spells, etc. and with gear appropriate for their level a party of 4 should beat a monster with CR equal to their level with no fatalities, and consuming only about 20% of their resources.

In this case with what information I have from your post (I know nothing of what the 20 hit dice gave him and how well the deity abilities complement them) I would guess his CR to be about 22 - 24.

Answer 3

In the D&D3 Encounter Level system, adding +2 Challenge Rating is roughly equivalent to doubling the number of opponents, and +3 CR is like tripling them. You can use this equivalence to estimate CR. If a tweak doubles your attacks and defenses, that's roughly like adding another creature, so it's worth +2 CR.

Adding Divine Rank 0 gives your boss roughly double hit points plus resistances, but it doesn't affect offense at all. That's worth at least +1 CR, maybe a little more. To give the players the benefit of the doubt, I would use this estimate or the Deities & Demigods guideline, whichever is higher.