Fast casting against another wizard in Ars Magica

One of those forum conversations that leads into a brief explanation of how the game mechanics resolve magic… The Mirror of Opposition spell reverses the effect of a spell, but not the target. So a spell to hurt a person might help them instead. It is a very interesting spell because it relies on so many of the meta-magic layers in Ars Magica, primarily around Muto Vim spells, but there are compounding questions in the details.

Pyrale wrote: Another question regarding spell manipulation : Supposing I see someone starts casting, and I decide to cast a mirror of opposition on his spell. Does my spell have to pierce the caster’s defenses ?

If not, do I have a window of opportunity if the spell has no time of travel, or can I only cast it on projectiles ?

My 2c which might be dodgy… I think you can cast it on anything, and the SG gets to decide what it does.

  • First you need to use the rules for identifying what Form the spell is, because Mirror of Opposition is different for each Form, but I guess you might know if the caster had previously been shooting away with missile spells and just assume you’re correct if your perception Check failed. The Perception check isn’t going to slow down the rest.
    … Perception + Awareness + roll vs. 15 – (effect mag). If the opposing caster has silent/subtle magic/deft magic it gets a lot harder.
  • Then if you are not cooperating with the other caster (fair assumption) you need to fast-cast the MoO:(form), as per p.159 of Ars about not being able to intercept the spell quickly enough. Its basically a -10 to casting roll with 2x extra botch dice, and you need the fast-cast Mastery option for the MoO spell.
  • Then your Penetration total for your casting roll (which includes that -10 mod) must beat the other caster’s Penetration total
    (i.e. spell CT + roll – Spell level + Pen ability vs same from the other guy).
  • The rules imply that you don’t need to penetrate the caster’s parma which is great, although if they were casting a spell with Range: Personal upon themselves I’d argue that you’d also need to penetrate their Parma for it to work too (?).
  • The caster must be in range of your MoO spell too (voice), unless you’ve got a funky version or using an Intangible Tunnel and an AC.
  • I don’t know what happens if they are fast-casting a mastered spell, but I think you still could fast cast to intercept and alter it (?).
  • You can’t alter Spont spells with MuVi which is a bugger but it makes sense why.

Easy! However…

  • If the first spell was fast-cast then the second magus can fast-cast one to defend/intercept, but it should come back to speed (point made here). Essentially the magic system seeks to avoid situations where magi just fast-cast-BLAST their way through all encounters.
  • As MoO and many other Muto Vim spells do not work against Spont magic, and also typically must penetrate against the target spell, and also usually only affect half their level, not many wizards are able to Spont cast Muto Vim spells in defence. It is possible, but very unlikely.

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