Design goal : To have a faith-based magic user who worked noticeably different from a wizard.
Also, to have clerical magic be more useful to worshipers of the same faith. In fact, have cleric magic be almost infectious in that regard.

Hit Points: 1d6

Class ability – Miracles

Cleric has a pool of dice of 1d8 per level. When using a miracle a number of d8s are rolled, one for each target the cleric wishes to affect. If a d8 roll comes up as a 1, the miracle (at least for that target) fails and the die is removed from the cleric’s dice pool. 2-7 result is the miracle works and die remains in dice pool. If an 8 (or higher) is rolled, an extra person can be affected by the miracle and the cleric gains a mystic mutation. Anytime a 8 or higher is rolled on any related miracle rolls the Cleric gains a mutation. Instead of manifesting it himself, the Cleric may pass the mutation onto one of the targets of the Miracle that has fewer mutations than he does. Each time the Cleric has a chance to pick another target he can choose not too and keep their Faith pool stable. If any roll in the chain gets a roll of 1, the Cleric loses a die from their Faith pool and the last target is not affected.

(So using a Blessing spell on the party’s fighter and rolling a 8+ on the d8 means the cleric can either gain a new mutation themselves or give the fighter a new mutation, either way the Cleric can roll a d8 to affect a second target. If that second roll is a 8+, once again the Cleric can gain a mutation themselves or grant it to the second target, and either way the Blessing can now affect a third target. Which means the Cleric rolls another d8 to see how it goes.)

Targets of miracles that have mystic mutations of the same power and faith add a +1 per mutation to the miracle roll that affects them. Likewise a Cleric may add to their rolls with holy water, incense, chanting for a turn before performing the miracle, and even some rare holy relics can assist.

If a 8 or higher is rolled for a miracle, another person can be affected by the miracle and the cleric rolls another 1d8 for the second target. If that result is an 8 or higher, the miracle can spread to another person, and so on. There is no upper limit to this, only practical limitations such as available targets.

The result of the d8 roll (modified by bonuses and penalties), plus the Cleric’s Wisdom modifier, is the amount of effect the miracle has. Hit Points for healing, damage for attacks, number of turns affected by fear effects, etc.

Miracle types:
Command – Cleric channels the power of their God into single word command. Burn, Halt, Flee, etc. One person/creature can be affected with each d8 rolled for the miracle.

Blessing – Cleric channels the power of their god to enhance someone. Limited to a single active option. Attacking, picking locks, climbing, etc. Can be maintained as long as the Cleric concentrates and chants. No other miracles or complex actions (fighting, swimming, climbing, etc) can be performed. However, the same Blessing can be cast again to cover more targets.

Abjuration – Cleric channels the power of their God to protect. One person/creature per d8 rolled can be affected and it lasts as long as the cleric concentrates. No other miracles or complex actions (such as fighting) can be done while maintaining an Abjuration, but the same Abjuration can be cast on more people with another use of the miracle.

Each faith should have a limited number of each type of miracle their clerics can perform. These should be set by the DM whenever a Cleric is created. Average should be 3 Commands and 3 Abjurations. Though some faiths may have more than one than the other. As long as they add up to 6 total. Every faith has at least one of each. The commands should double as Magic Words, though Clerics cannot teach them to others because they speak it in their own language and the power of their god makes it a Magic Word.

A cleric may also perform holy ceremonies, which are very much situational. A ritual requires a large amount of resources and components and may take anywhere from several days or several months to accomplish.

Cleric based mystic mutations can alter already existing mutations a person has. Can’t get rid of them, but if a person has mutations from another source a new mutation can instead ‘purify’ and change a current mutation into one of the Cleric’s faith. So a Cleric with a divine power uses a miracle on a wizard with demonic mutations, it can turn one of the demonic mutations into a divine mutation instead of adding a new one.

Chance for immortality:

Clerics that perform some great service for their patron may get a chance to keep their individuality when they reach nine mutations and become a manifestation of divine power. This would require either converting an entire hostile city (with proper temples built) or retaking some territory that had fallen to enemy forces and consecrating it as the faith’s territory once more. Which would include holding it with followers, not just clear it out and bless it then move on.