NHF – new heroes for Heroes V 3.1.

Auto-aim is needed; otherwise, there will be a bias in favor of hits, exceeding 30%. We limit misses, but not hits? That's not right.

I need the most accurate and complete information about the special ability to suggest the right stabilization option.

Okay :)
Everything is already described...

The chance of a spell appearing is 30%. Moreover, there is a check for duplicates. That is, if a duplicate appears, it is regenerated to another one 10 times, with a probability of 100%. If it doesn't appear, then it won't appear.
So, in theory, the chance of appearance is slightly less than 30%.