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I didn't see any info...

Frist level Changeling, Mages Assistant, gets a +1 to their Corruption threshold.
Stats giving him a +2 Proficiency Bonus and +3 Spellcasting modifier, so a Corruption Threshold of 11 = ((2 + 3) x 2) +1

Choose Guiding Bolt as Favored = 1 Temporary Corruption per cast

He will exceed his threshold after casting the 12th Guiding bolt (4d6 damage, each)
This requires a DC1 test? Roll a 1 on a d20 and he may get a 24hr cosmetic mark?

Some reduce Temp Corruption, so he can keep casting
Or if he continues to cast, irs just DC2, DC3, etc?

The rule is on page 38:

"If your total Corruption (temporary plus permanent) exceeds your Corruption Threshold, you must make a Corruption check, to see if you can bear so much darkness. Roll a d20. If the result is equal to or less than the amount by which your current Corruption exceeds your Threshold, you will gain a mark of Corruption. Roll 1d20 on the Marks of Corruption table to the right. The mark fades 24 hours after your total Corruption drops below your Corruption Threshold."

So you first make a Corruption check (DC 1 as you put it). If you fail, you roll on the table on the right for the effects of the mark. The DC of the roll increases as your total Corruption increases. It is possible that a Mark of Corruption reduces or resets your temporary Corruption.

I guess my question, or rather point is... with classic 5e, at first level, you get 1 first level spell slot., and can only cast once per long rest.. In Symbaroum 5e, you are MUCH MUCH more powerful, since you can cast 11 times without any corruption roll, rather than once

Unless I got this wrong... For my games, I'm going have to make casting harder, more risky then classic 5e.

In classic 5e, Wizards can cast 2 lvl 1 spells, plus Arcane recovery, thus 3 first level spells per long rest.
In RoS, Mystics do not lose all temporary Corruption on a long rest (only on extended rests). They can cast more spells, especially favored ones, before reaching the Corruption threshold, but they can't keep casting 11 1st lvl spells each day during the adventure, they have to keep some juice for the following days.