There are quite a few feats that are provide special benefit to the Killer Gnome. These feats provide more benefit to him than to most other spellcasters. You should remember, however, that a Shadowcraft Mage is even more feat strapped than other characters, as four of his feats are set in stone from the beginning (Spell Focus [illusion], Earth Sense, Heighten Spell, and Earth Spell.) That leaves a Sorcerer-based character but three remaining feats.
Choose wisely.
As is the norm for these guides, I'll be using font colors to facilitate quick skimming.
Horrible: Known by the color of the feces they're made from, I'm only going to list a few of these, and then only because they seem to be traps.
Average: These options are situationally useful or just not good enough to be worth taking until you've exhausted the better options. Sometimes, they're prereqs for good things, though, so ...
Tax: These are the things you take to get your abilities to work in a non-suck way.
Good: Recommended choices. These will help you, but sometimes require support to make good or are only good after you reach a certain point in your progression.
Great: It's dangerous to go alone. Take these, because they're always good and need no support.
Busted: And lo, there was a great, deep quake throughout all the lands, as though the game itself had broken.
Ability Focus: Ability Focus gives +2 to the DC of a chosen special attack. Now, even if you don't allow Ability Focus (spells)--which is arguably legit--it's still kosher to choose Ability Focus for shadow illusion or
silent image, and that's still +2 DC for one feat.
Arcane Mastery: The ability to take 10 on caster level checks (even under stress - check the errata) is great for anyone who uses a large number of spells that allow SR. As all shadow illusions allow SR, this is obviously a good thing. With nothing else to boost caster level, a 20th level Shadowcraft Mage can automatically beat SR 30 with his spells; and Earth Spell makes even higher levels of SR meaningless.
Arcane Thesis: Arcane Thesis has the potential to add significant power to the Shadowcraft Mage. The feat is normally somewhat restrictive, needing to be applied to a single spell. This is no drawback to the Shadowcraft Mage, whose bread and butter actually is one spell,
silent image. A +2 bonus to caster level is always helpful for the increased duration, resistance to dispelling, and ability to punch through SR. The reduced metamagic cost, while specifically not applicable to Heighten Spell, applies to everything else. So load up on low cost (+1 and +0) metamagic feats via whatever method you want (I suggest Incantatrix) and blast away with silent, stilled, enlarged, widened, and doubly extended Shadow Illusion at no slot-level adjustment.
Easy Metamagic: This feat from Dragon #325 functions like (and stacks with) Practical Metamagic (see below) except that it can be acquired by any spellcaster and has the [metamagic] tag, so it can be had via a bonus feat. So strictly better.
Enhanced Shadow Reality: Adds 20% extra quasireality for all damage on your shadow illusions. The only difficulty is that the feat is from Dragon (#325), so getting access to it may be a trial. Remember, this is only for damage, but it does help you get to 100% quasi-reality with lower-level slots, freeing up higher-level slots for more impressive effects. Statistically, this represents a 5-15% increase in expected damage, which means it's probably not worth the opportunity cost.
Extend Spell: The quintessential buffing metamagic. There isn't a better +1 SLA metamagic feat in the books. The fourth level Shadowcraft Mage specifically stacks with this feat, to boot, giving you durations that make your enemies cry. You're still likely better off just using metamagic rods, but the option is valid. Additionally, you can further stack this with the 5th level gnome Illusionist substitution level to get those durations to a total of TIMES EIGHT. (Yes, duration doublers stack the good way.)
Greater Spell Focus (illusion): You get more bang for your buck from illusions than anyone else. Your save DCs should be as high as possible, and every +1 helps, but this feat is pretty much just gravy, as it is only required for entrance into the Shadowcrafter PrC (see below).
Melodic Casting: Every Bard-based ScM with a significant inspire courage bonus will want this feat. Don't even bother arguing with me. It's like Natural Spell for Bards, making it so they never have to choose between singing or casting.
Metamagic Feats not Previously Mentioned: If you have the feats free and have Residual Magic (discussed below) you can get some massive power out of Quicken Spell, Twin Spell, Repeat Spell, and Energy Admixture. If you're an Incantatrix, you're of course picking up Persistent Spell, in which case you'll also want Invisible Spell -- because you
are persisting
arcane spellsurge, right? For more on metamagic feats and getting the most out of them, see
Metamagic and You: A Thesis.
Metamagic School Focus: This feat allows one to apply any metamagic feat to a spell of the appropriate school at reduced cost thrice per day. So three times per day you can mimic a spell a level higher than normal, even past what Earth Spell alone would allow. More controversially, some argue that the feat's 3/day lowering can all be applied to a single casting of a spell, allowing for accelerated access to mimicked effects and early PrC qualification. Seems bogus to me, but if you can get it to float, this becomes
great.Practical Metamagic (Heighten Spell): Practical Metamagic reduces the spell level adjustment of the selected metamagic feat by one, to a minimum of +1. So this means that a
magic missile heightened to third level occupies a second level slot.* Heightened to ninth, it occupies an eighth level slot. So we can mimic a spell of level X in a slot of level X, as long as X is less than nine. The limitation exists because Heighten Spell can only increase a spell's level to a maximum of ninth.
By way of this feat and Earth Spell, a Sorcerer can mimic spells of one level
higher than the slot used (except for ninth level slots, which, as mentioned above, are unaffected by the Practical Metamagic adjustment). So he can cast a 100% real
meteor swarm from an eighth level slot, and it will be treated as a tenth level spell for all purposes, including save DC.
*
Note that it is occasionally debated whether the use of Heighten Spell constitutes a spell slot adjustment, so a DM may argue that Practical Metamagic and Easy Metamagic do not work with Heighten Spell. This is a failure of exegesis and basic reading comprehension, but you have been warned. ;)Rapid Metamagic: For Sorcerer, Beguiler, and Bard ScMs this eliminates the casting time penalty of applying metamagic. A very good feat that has been too long in coming. It is also quite important for spontaneous casters who are not Sorcerers (who have an applicable alternative class feature in PHB2), allowing them to retain mobility in combat.
Reserve Feats: Yes, all reserve feats are bad. A spellcaster has better things to do with his time. The reserve feats don't have the power to be significant against strong enemies, and you might as well let the Fighter feel useful every now and then by letting him take on the mooks singlehandedly. Seriously, stay clear. Unless you intend to retrain them later or Dark Chaos Feat Shuffle them away. Then go ahead.
Shadow Weave Magic: As I mentioned before, spells' save DCs are important for most Shadowcraft Mages, and another +1 can't hurt. The important thing, however, is that it opens access to several other feats that can be gained through one level in Shadow Adept, and all of them synergize quite well with the class features of Shadowcraft Mage.
Signature Spell: Signature Spell can be a boon to prepared casters like Wizards, as it allows them to load up with nothing but utility spells. By choosing
silent image for the feat, the Wizard can then spontaneously convert any prepared spell into a heightened
silent image and alter that spell with Shadow Illusion. The Spell Mastery requirement holds this one back. There's a better alternative coming up soon (hint, it's called Uncanny Forethought).
Spell Penetration: Spell resistance is your bane. Statistically speaking, however, you get a much more significant benefit from Arcane Mastery. It's not like having this feat will hurt you, except inasmuch as taking it means you're not taking a feat that actually matters.
Talfirian Song: This feat allows Bards to trade uses of bardic music for extra spell levels, and--unlike Metamagic Song--it isn't capped by the maximum spell level you can cast. It's a do-it-all feat that gets you early entry and early access to high-level effects. You do have to be human, but see later in the guide about how to enter Shadowcraft Mage as a non-gnome.
Uncanny Forethought: Now
this is how we spontaneously cast things and make the Sorcerer cry (even more than he or she already was). While it still requires Spell Mastery, the versatility it affords is simply astonishing. It even has its own built-in cheese that lets you get around some spells' long casting times. For instance, it could let you cast
geas as a full-round action, making it an actual combat option. Considering it even shares same prereq as Signature Spell, there's no reason to take that rather than this.
A Digression on Pathfinder Spell Perfection: Spell Perfection's gotta be your go-to if you've got access to it. The feat doubles numerical bonuses from other feats (e.g., Spell Focus) and allows you to apply one metamagic feat for free (i.e., no adjustment to casting time or slot level). That shit's busted enough on its own, but think what it means in Pathfinder. Add the first Gnome Illusionist sub level (see below) to make silent image a cantrip, and you suddenly can cast all these spontaneous shadow illusions at-will, all day long. Add Earth Spell and Arcane Disciple, and you can cast shadow miracles all day long.