To understand better, I tried to calculate the Corruption mechanics for Mystics, based on 5E spellcasting rate.
It is of course just an example, and a real game would not look like that.
I considered a 5th level Human Mystic that casts 4x 1st level, 3x 2nd level, 2x 3rd level before getting a long rest. Cantrips used are favored, and the Mystic has 3 favored leveled spells + 3 unfavored spells, that he uses 50% of the time (rounded down).
That represents a total of : 2x1 + 2x(d4+1) + 2x2 + d4+2 + 3 + d4+3 = 26 pts corruption
Artifact Crafter
Thanks to Talisman Crafter, with 3 talismans, this approach halves the effect of the 3 unfavored spell.
Thus he gains only : 2x1 + 2x(d4+1)/2 + 2x2 + (d4+2)/2 + 3 + (d4+3)/2 = 19 pts corruption.
With +4 Ability, +3 Proficiency, the corruption Threshold is 14.
With one Short Rest the Mystic could stay below the Threshold by spending 1HD. (-6 points)
Later a Long Rest he will decrease the Corruption to 7 points (-6 points again).
Self Taught
This Approach knows less spell. 3 cantrips + 5 leveled spells as a level 5 + 2 rituals.
A 1st level + one 2nd level are not favored, the 3 others are.
The Self Taught has interest to learn low level spells and to cast them at higher level, especially for unfavored. Let us suppose he casted a 2nd level at 3rd level, and a 1st level at 2nd level in the list.
Thus he gains only : 2x1 + 2x(d4+1) + 2x2 + (d4+1) + 3 + (d4+2) = 23 pts corruption.
With One Short Rest and 1HD spent on Corruption, the Self Taught will manage to stay at 17 pts of Corruption, but above Threshold.
Hel will roll 3 times for Marks of Corruption, and the probability to fail such a roll is : 27%
However, at the cost of 3 permanent Corruption, the Self Taught has been able to learn Find Familiar + Blood Bond. He transfers permanent Corruption up to 2 to a familiar, then summons a new one. So he should be able to transfer permanent corruption during the adventure to a familiar, and temporary corruption to 0 at the end of the adventure. Spending Thalers, he starts the next adventure with a new familiar.
Another approach could be to use Purging Fire as a Ritual to remove all Permanent Corruption at the end of an adventure. That’s a huge advantage compared to other approaches.
Sorcerer
Thanks to the Shadowed Slope, the sorcerer gains : 2x1 + 2x(d4) + 2x2 + d4+1 + 3 + d4+2 = 22 pts corruption.
With One Short Rest and 1HD spent on Corruption, the sorcerer will manage to stay at 16 pts of Corruption, but above Threshold.
However, thanks to The Darkened Path feature, the probability to fail the Corruption rolls is low :
1,25% only.
The sorcerer has no spell to reduce permanent corruption.
Staff Mage
The Staff Mage has no feature to reduce temporary corruption.
He will receive the 26 points of corruption minus the Short Rest = 20 points in final.
He will have to roll 6 times, and the probability to fail at least one roll is : 70%
The Staff Mage can transfer 3 points of Permanent Corruption to his staff.
Also, at the cost of 3 permanent Corruption, the Mage has been able to learn Find Familiar + Blood Bond. He transfers permanent Corruption up to 2 to a familiar, then summons a new one. So he should be able to transfer permanent corruption during the adventure to a familiar, and temporary corruption to 0 at the end of the adventure. Spending Thalers, he starts the next adventure with a new familiar.
Symbolist
The Symbolist has 6 runes available. It is quite expensive for this approach to cast high level unfavored spells (level x2 corruption).
That represents a total of : 2x1 + 2x2 + 2x2 + 4 + 3 + 6 = 23 pts corruption
With One Short Rest and 1HD spent on Corruption, the Symbolist will manage to stay at 17 pts of Corruption, but above Threshold.
Hel will roll 3 times for Marks of Corruption, and the probability to fail such a roll is : 27%
However, at costs of 3 permanent Corruption, the Symbolist has been able to learn Find Familiar + Blood Bond. He transfers permanent Corruption up to 2 to a familiar, then summons a new one. So he should be able to transfer permanent corruption during the adventure to a familiar, and temporary corruption to 0 at the end of the adventure. Spending Thalers, he starts the next adventure with a new familiar.
Theurg
The Theurg has no feature to reduce temporary corruption.
He will receive the 26 points of corruption minus the Short Rest = 20 points in final.
He will have to roll 6 times, and the probability to fail at least one roll is : 70%
However the Theurg has the possibility to use Purging Fire as a Ritual to remove all Permanent Corruption at the end of an adventure. That’s a huge advantage compared to other approaches.
Troll Singer
The troll singer can cast up to 6 levels of spells without Corruption. Let us suppose he will use this feature to cast the 2 3rd level spells of our list. He would then have to make Charisma saves for the remaining spells, at +7.
The probability of success for DC14 is 91% for favored, 70% else.
For DC 16, 84% for favored, 60% else.
The average value of permanent corruption he ends up with : 2x1x9% + 2x1x30% + 2x1x16% + 1x40% =1,5 pts of permanent corruption.
The Troll Singer has no spell to reduce permanent corruption, but on the other hand he does not gain temporary corruption.
Witch
The Witch has no feature to reduce temporary corruption.
He will receive the 26 points of corruption minus the Short Rest = 20 points in final.
He will have to roll 6 times, and the probability to fail at least one roll is : 70%
However, at costs of 3 permanent Corruption, the Witch has been able to learn Find Familiar + Blood Bond. He transfers permanent Corruption up to 2 to a familiar, then summons a new one. So he should be able to transfer permanent corruption during the adventure to a familiar, and temporary corruption to 0 at the end of the adventure. Spending Thalers, he starts the next adventure with a new familiar.
Wizard
The wizard has no feature to reduce temporary corruption.
He will receive the 26 points of corruption minus the Short Rest = 20 points in final.
He will have to roll 6 times, and the probability to fail at least one roll is : 70%
However, at costs of 3 permanent Corruption, the Wizard has been able to learn Find Familiar + Blood Bond. He transfers permanent Corruption up to 2 to a familiar, then summons a new one. So he should be able to transfer permanent corruption during the adventure to a familiar, and temporary corruption to 0 at the end of the adventure. Spending Thalers, he starts the next adventure with a new familiar.
I find this Corruption system quite effective : it could have been interesting, with the mystic having to think twice before casting a spell, and define a strategy for the whole adventure. « Do I keep a reserve for the big boss, and take a risk now ? ». And in the end, the Corruption will get him.
But it seems there are too many loopholes in this system : Theurgs, Self Taught (who can cast Purging Fire many times until their Permanent Corruption is 0), and the combo Find Familiar + Blood Bond which neutralizes the problem as long as you have a few hundreds of Thalers. This is quite a lot of exceptions…
That seems quite the opposite for the combat classes : all of them are more powerful than 5e, which is already highly heroic system. All of them have up to 4 attacks, approaches with permanent advantages or dodge…
In the end I have the feeling this won’t feel enough as a "Dark Fantasy" setting, players will choose obviously Mystics with ways to eliminate Permanent Corruption and therefore will cast spells almost as in 5e, and the fighters will hack their way out of problems more easily than in 5e. Of course the limited number of Hit Dice will slow them down, but I don’t think this is enough to make us feel we are in a Dark Fantasy setting where the heroes will be fearing their environment and opponents.
What is your feeling dear reader, especially those who have played a lot Symbaroum ?