Everything's a cantrip

[Raishoiken]

I've seen brief mention in a few threads of shadowcraft mages combining signature spell, heighten spell, residual magic, and a few other steps to use signature spell to cast heightened silent images from 1st level/cantrip slots. I think it's meant to allow you to take heightened silent image as your signature spell or something. Does anyone know if this is actually supposed to work?
Also, regardless if it does or not, are there other ways this is possible using anything else? Or any reduction in level really

[Ruethgar]

Sanctum Spell reduces effective level under all circumstances that is not being cast within the sanctum(as opposed to the common misunderstanding of -1 level when cast outside the sanctum). There are a few uses for that.

Dragon Magazine 313 has the Mind Mage who can theoretically cast 9th level spells out of first level slots if they can get the 60ish manifester level needed. Spellhoarding dragons(same magazine) can then catch those spells onto scrolls for others to copy(Wizard)/steal(Spelltheif)/mindrape(StP Erudite).

Pathfinder has the Whispering Way organization who have the potential to learn all necromancy or fluffably necromantic spells up to two spell levels lower.

Shadowcraft Magi can indeed cast highten silent images that are reduced, by various means, back down to cantrip level in order to more cheaply ‘cast’ higher level conjuration and evocation spells.

[TalonOfAnathrax]

I've seen brief mention in a few threads of shadowcraft mages combining signature spell, heighten spell, residual magic, and a few other steps to use signature spell to cast heightened silent images from 1st level/cantrip slots. I think it's meant to allow you to take heightened silent image as your signature spell or something. Does anyone know if this is actually supposed to work?
Also, regardless if it does or not, are there other ways this is possible using anything else? Or any reduction in level really
This handbook (http://minmaxforum.com/index.php?topic=16433.0) has a complete, detailed and sourced explanation for how exactly that trick is supposed to work. It also includes the "get infinite cantrips and level 1 spells" Shadowcraft Mage Wizard trick, which allows infinite Miracles from cantrip slots.

[AvatarVecna]

The typical method for a Shadowcraft Mage to get Silent Image as a cantrip spell is the Gnome Wizard racial ACF that does literally exactly that thing I just said. But I guess metamagic shenanigans with Arcane Thesis, Spell Perfection, Sanctum Spell, and the like can also pull it off, or even combine with it pretty well.

[magic9mushroom]

I've seen brief mention in a few threads of shadowcraft mages combining signature spell, heighten spell, residual magic, and a few other steps to use signature spell to cast heightened silent images from 1st level/cantrip slots. I think it's meant to allow you to take heightened silent image as your signature spell or something. Does anyone know if this is actually supposed to work?

The trick is:

1. Cast Silent Image (cantrip via Gnome Illusionist) heightened to 10th level with Heighten Spell/Earth Spell. This eats a 9th level slot.
2. Next round, cast a non-heightened Silent Image out of a cantrip slot (or out of a higher-level slot because you put other metamagic on it), and it gets heightened for free via Residual Magic.

Rules are a little hazy on how Residual Magic and Heighten Spell interact, because of the way Heighten works, but aside from that this is literally what Residual Magic does. It's a tactical feat that, if you cast a spell twice in consecutive rounds, lets you apply a metamagic feat you applied to the first to the second for free.

The reason Residual Magic is rarely seen outside of Shadowcraft Mage builds is that it has to be the same spell, and spamming one spell all the time cuts down on your versatility - unless you're a Shadowcraft Mage, whose entire schtick is spamming Silent Image and haxxoring it into almost anything via Heighten Spell/Earth Spell/Shadow Illusion. It also can't transfer more than one metamagic, which means it's typically less efficient than the various other ways to cheapen metamagic for orb/ray metamagic-stacking builds.

Also, regardless if it does or not, are there other ways this is possible using anything else? Or any reduction in level really

Define "this" and I can give an answer.

[Kalkra]

Regarding Residual Magic, Shadowcraft Mages often use the ambiguity to Heighten a spell one level, and then use Residual Magic to Heighten it nine levels for free. That's just the easiest way for that build to get free metamagic, but any of the many tricks would work for that.

Other than Mind Mage, there's a PrC called Ambient Tempest from Bestiary of Krynn (Dragonlance, 3rd party) that lets you cast spells a level lower by applying either reverse Extend, Widen, or Lengthen, as appropriate (max -1 level).

[magic9mushroom]

Regarding Residual Magic, Shadowcraft Mages often use the ambiguity to Heighten a spell one level, and then use Residual Magic to Heighten it nine levels for free.

That's not in accordance with the text of Residual Magic. It doesn't say "apply a metamagic feat from the first casting to the second casting for free", it says "apply any one of the metamagic effects from the first casting to the second casting".

You don't need to cheat that way anyway. Echoing Spell is very fun for an SCM due to Earth Spell raising caster level.