ReVi(Mu) 40 - Voice | Conc | Ind | - Spell Box
If penetration score of the Spell Box effect is greater than the spell being boxed and the targeted effect is of level no higher than that of the Spell Box, you can keep the spell in the box while you concentrate & release it onto any target you wish, by the same targeting means as originally. The boxing spell needs to take root while the other is being invoked as you cannot contain existing effects. The boxed spell retains the sigil and the penetration score of the original casting.
(Voice +2 | Conc +1 | Ind +0 | Base 40 (General +5), +2 Change of target)
Here's what I'd identify as the immediate 'exploit' scenario (or possibly the as-intended?):
cast Pilum of Instant Death in the comfort of my own sanctum on some random in-sanctum target. Accrue as many bonuses to casting total as I possibly can.
cast 'spell box' on my own pilum (or better yet my friend's pilum, thus avoiding the concentration rolls) to box it and keep it for later
when in the field, unbox as needed
The most significant factor here is going to be auras. If I can cast my spell in my sanctum with the magic aura bonus, then move it into a hostile aura and unbox - I'm essentially bypassing the aura penalties. I'm also bypassing all the usual grief of creating a magic item (notably the time investment), which is the typical way one gets around this barrier. It also lessens the impact of flaws like 'weak enchanter' since it provides a work-around solution.
So it's certainly an option as a spell, as long as the saga is happy with the intended use and what other areas of the rules it overshadows. I wouldn't say it is necessarily overpowered - and it's certainly thematically appropriate.
Wouldn't this Target the item in your sanctum and not the bandit/knight out in the field whom you might now want to Target? It would be easier to make a wand of Pilum of Instant Death so you can target what you want.
Where it could work would be for protection spells. Still the concentration makes me want to say that this would be done better in an enchanted item.
I'm not too sure about the spell as written. It seems it does two things: a primary effect of "spell-holding", and a secondary effect of "target-changing". My main problem is with "target-changing": it's a change that takes effect AFTER the main spell is formed, while it is held, whereas Muto Vim can't affect a spell after it's been created. Without the "target-changing" I'd be quite ok with it!
It is true that MuVi Magic needs to change an effect while it is still being cast, or during that moment which means that the pillum of fire that you box in has to be boxed it while your target is within target & then when released from the box, it would attempt to target it.
My original use of this effect was to better manage interactions in-between magi. One casts a spell & you box it. Then you can redirect the effect to a harmless target or back to the originator (needs to be chosen at the same time as you box the effect).