I’m trying to understand the rules for Casting Tablets as intended, specifically as it relates to rituals.
Covenants, on page 90, says:
Rituals may be cast from tablets. It is possible, but dangerous, for a magus with a tablet to lead a Wizard’s Communion. A botch by any member of the group causes the leader also to botch.
First off, it says Wizard’s Communion here, but I assume it means Wizard’s Vigil since that is MuVi spell useful for Rituals.
Second, I’m not clear on what this means. Is it saying that its possible to cast a big ritual spell from a casting tablet with your buddies casting wizard’s vigil to make it easier? Or is it saying that its possible to cast a big ritual spell that you know with your buddies casting wizard’s vigil from a casting tablet to make it easier? Or maybe everyone is using a casting tablet?
Third (and the real difficulty I’m having) I’m not sure why this is called out as particularly dangerous. Casting tablets don’t give additional botch dice. They do require a stress die, but so do ritual spells in general. It seems like a great way to cast a powerful ritual spell is to get a casting tablet of that spell and get some friends who know wizard’s vigil to cast it for you.
For instance, if I have a casting tablet of Shadow of Life Renewed and three friends who can cast a decent level Wizard’s Vigil then my target for the Shadow of Life Renewed is 25. Which means I can’t actually fail to cast the spell, even though I have no Creo or Corpus, unless I botch (since the casting tablet lets me cast it successfully even if I miss by 30). I have a less than 4% chance of botching. That’s not something I’d want to do every day, but it seems very powerful.
I think they spoke of two things. The dangerous part is the Communion or Vigil from tablets, since everyone is using a stress die, increasing the likelihood that a member of the group, and therefore the entire group, will botch. This lies in opposition to the group using formulaic Communion / Vigil. Remember that Communion / Vigil comes with extra botch die and collective backlash.
That is correct; Wizard’s Vigil didn't exist at the time Covenants was written.
Let's look at the description of Wizard's Communion/Vigil, and substitute "Aegis" for "specified spell" to make it easier to understand:
"Only one extra magus may join the Communion for each 5 levels of the Aegis being cast. One of the magi in the group must also know the Aegis, or cast it from a Casting Tablet."
I think leading a Wizard's Communion/Vigil means that you're the guy who knows the actual spell being cast (the Aegis).
Wizard's Communion/Vigil only says:
"This method of casting adds one botch die per magus participating [to the Aegis roll]. If the Aegis botches, all participants gain Warping Points and must check for Twilight if they gain two or more."
I read that as meaning that if someone botches his Vigil when casting a Ritual the regular way, then the effective level of the Wizard's Vigil (the total levels of successfully cast Wizard's Vigils) might be too low, so the leader unexpectedly doesn't divide the level of the Aegis by the number of participants. That means his penetration is much lower than intended, or he fails to cast the spell entirely. In any case, the Vis used for the Aegis is probably wasted.
When you use a casting tablet, a "botch by any member of the group causes the leader also to botch", so when any participant botches his Vigil, the leader also botches the Aegis - his Casting Total is 0, he likely misses the spell level by more than 30 points (remember that Penetration has to be built into the spell) and gains Warping equal to the magnitude of the Aegis.
On a different note: For me the real reason why casting an Aegis from a tables is dangerous is independent of the use of Wizard's Vigil: You usually have Mastered the Aegis and are casting it in relaxed circumstances, so there are no botch dice. A spell from a tablet cannot be mastered, and you're using lots of vis, so you have a substantial chance of botching every time you cast.