Mouseferatu, I tend to lurk only at this board, but I couldn´t resist this topic. I loved the concept of the shadowcaster but was disappointed with the mechanics. I applaud the initiative to debate your material and I hope my comments below provide a positive feedback
Now, this doesn't go as far as some of you have requested, but it's where I'd like to start. If some of you would like to volunteer to run a few playtest adventures with these changes in place, I'd very much love to know how it goes.
It’s possible that in January I’ll play an one-shot of a 20th lvl adventure. Originally, I intended to use a warlock/evoker/eldritch theurge, but I talked with the DM and he agreed to a modified shadowcaster. If this session happens, I’ll post the results.
Grant bonus mysteries per day based on Int. These would work just like bonus spells. For instance, if your Int is 14, you can cast one extra mystery of 1st-level equivalent and one of 2nd-level equivalent per day. (Note that each mystery does give an equivalent level, even though you don't learn them by level.)
I really like the idea of bonus mysteries per se and tried to put some thoughts on it. If the calculations below are wrong, please correct me

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Ex: A 20th lvl shadow caster has elite array (15 Int, 14 Cha), proper equipment (tome of clear thought +5, headband of intellect +6) but didn’t invest any of the ability points granted at 4/8/12/16/20, so the M.A.D. will have it’s full effect on INT, which will have a total score of 26 (+ 2/2/2/2/1/1/1/1/0 mysteries/day). I confess I didn’t understand the concept of a “floating” bonus, so I’ll stick with the extra mysteries applied to each level normally. So, assuming this shadowcaster learned two mysteries per mystery level, at 20th he has the following number of uses:
1 (Su): 3 x mystery, 3 x mystery,+2 bonus
2 (Su): 3 x mystery, 3 x mystery, +2 bonus
3 (Su): 3 x mystery, 3 x mystery, +2 bonus,
4 (Sp): 2 x mystery, 2 x mystery,+2 bonus
5 (Sp): 2 x mystery, 2 x mystery, +1 bonus
6 (Sp): 2 x mystery, 2 x mystery, +1 bonus
7 (Spell): 1 x mystery, 1 x mystery, +1 bonus
8 (Spell): 1 x mystery, 1 x mystery, +1 bonus
9 (Spell): 1 x mystery, 1 x mystery
For a quantitative analysis only, this shadowcaster will be able to cast, daily:
8/8/8/6/5/5/3/3/2
IF the same shadowcaster wholly privileged INT for the bonus ability points, his final INT score would be 31 (+ 3/3/2/2/2/2/1/1/1 mysteries). The total uses of mysteries per day would be:
9/9/8/6/6/6/3/3/3
Comparing with a wizard with 15 (initial) + 5 (ability points) +5 (Tome) +6 (headband) may cast:
7/7/6/6/6/6/5/5/5
I know this is hardly a reliable comparison and I don’t want to equalize the shadowcaster with the wizard (a class that IS powerful, to say the least). But I think there is a valid point, considering the shadowcaster has a smaller repertoire than the sorcerer, which has more spell slots than a wizard. HOWEVER, I think the “high INT” shadowcaster has a pretty decent number of uses/day which can make some compensation even though the wizard has the advantage on the three highest spell levels.
Conclusion: The bonus mysteries are a necessity, but IMHO will only compensate if the multiple attribute dependence is somehow alleviated.
Suggestion: Conceptually, I like the need of INT + CHA to employ the art of shadow: one to learn the subtleties of shadowcraft and the other to force the shadow to coalesce to your whim. However, instead of dividing the Save DC and the Extra Mysteries between two ability scores, apply both to ONE of these abilities, while the other remains responsible for the maximum mystery level possible to cast (i.e. needs 10+ Mystery level). This also solves the low save DC problem without reducing the shadowcaster to a single-attribute spellslinger. The ability not responsible for the DC and extra mysteries would still be important enough to guarantee the 2nd or 3rd highest attribute number (so a 13/14 with a +6 wondrous item grants access to the highest mysteries). I’d lower the final base Fort bonus to +6, though.
Regarding WHICH ability would be responsible for bonus mysteries and Save DC, I recommend CHA for two reasons. First, I think INT has enough mechanical attractiveness on its own (skill points), so a player wouldn’t complain to much for not upping CON, while the expense of Cha just to fulfill high mystery requirements might cause grumbling ;-). Second, for fluffiness, I believe CHA becomes a more interesting choice for shadow channeling and empowerment than INT
In sum: I agree with the bonus mysteries’ idea, except I’d prefer Charisma and but maintain it in charge of the Save DCs.
Eliminate the rule that says you have to take mysteries in a given Path in order. If you want to jump around, so as to broaden your versatility, you can. You must still have at least one mystery of any given level equivalent (1st, 2nd, etc.) before you can get a mystery of the next higher level within a type (Apprentice, Initiate, Master), but they need not come from the same Path =)
I agree 100% on that. Respectfully, the worst trait I saw on the shadowcaster was the strictness of the paths. Eliminating this handicap goes a long way!
Eliminate the rule that says you get a bonus feat equal to half the number of paths you have access to. Instead, you get a bonus feat equal to the total number of Paths you complete. Thus, while you are no longer required to take the entirety of a given Path, there's still encouragement to do so.
Agreed. It also gives meaning to the paths beside their natural coolness
Allow the shadowcaster to swap out mysteries, like a sorcerer does spells, with the caveat that if you suddenly "un-complete" a Path, you lose a feat as well.
Well, I’d support this idea, but not with the possibility of “losing” feats. I really don’t like that, sorry. Personally, I’d stick with the absence of “mystery-swapping” as, IMHO, it’s possible to plan ahead the mystery progression without much suffering.
Once your Apprentice Mysteries become supernatural abilities, change the save DC from 10 + equivalent spell level + Cha to 10 + 1/2 caster level + Cha. This makes them useful even against high-HD opponents, and follows the pattern for other supernatural abilities.
Personally, I’m not too fond of changing the mechanics of DCs in a class. Perhaps the suggestion above regarding a M.A.D less severe could solve the problem?
And BTW, the warp spell mystery shouldn't say that it allows a Will save. The caster level check determines success or failure.)
I agree 100%
Final Considerations:
1) Regarding shadowcaster feats, I have a question: Do Ability Focus/Empower Spell-like Ability/Quicken Spell-like Ability feats affect Initiate Mysteries or one Master Mystery which has been chosen by the Favored Mystery Feat?
2) Besides the problems discussed above, I believe there’s a last “underpower issue”: the class lacks real offensive options from character levels 2-4. While at 1st level the shadowcaster can count on Arrow of Dusk and Mesmerizing Shades, from then on until level 5 (i.e. killing shadows becomes available), the character suffers from lack of good mysteries, especially from 2nd level.