Myth is heavy into optimization and in the eyes of most CharOppers, shadowcaster is weak. But if you play smart, it’s actually a fairly good class. I know, as I played one for a while in a PbP here. If you want, I can dig up exactly where I joined that game and my character sheet for it.
In play, I did a bit of battlefield control with Black Fire and Clinging Darkness, plus some direct-hitting with a Reach Umbral Touch (the Slowing rider effect saved the party in one nasty fight).
As for the design and double-checking math, I’m happy to help. And welcome to ENWorld.
Here’s what I found needs fixing, after giving your sheet a quick look.
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It looks to me like you’ve spent way more than the 36 points. Even putting your level-up points into DEX to get the 18 still has way too many points spent. So either you’re leaving out magic items or you’ll need to drop some of those scores.
As an 8th-level character, you can increase any of your ability scores by 2 (at 4th and at 8th level, you can give out a +1 advancement bonus), which I don’t see having been spent.
You get an extra language known for each point of Intelligence bonus you have.
Your Hit Points get to add your CON bonus on each level.
Obviously, you haven’t bought any armor. That said, no armor is precisely why your flat-footed AC can’t be 22. Flat-footed is your AC without any Dexterity bonus. Touch is rather the opposite; you only count Dexterity (and some other magical bonus types).
You are missing a feat. You get a feat at 1st, 3rd, and 6th levels, plus a bonus for being Human (at 1st-level, so make sure you qualify for it then) and a bonus at Shadowcaster 2 (drawn from a specific list).
Oh, and Myth will want it in her special format.
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Now, as for boosting the character...
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Regarding Feats – Iron Will likely isn’t that important a feat to keep. You have a good Will save (and after fixing stats, an average Wis) and a Cloak or Vest of Resistance will help boost it a bit more.
Are you especially set on Shadow Familiar? It doesn’t add much mechanically and could be replaced with a Metashadow feat. The same could be said of Path Focus and Iron Will.
You obviously need to spend your starting gold. A +1 Mithral Chain Shirt of Twilight will give you +5 AC, no Armor Check Penalty and no Arcane Spell Failure chance. A Mithral Buckler will do similar. Other protective magic items may be worth your time.
Also, magic items to boost your CHA are important. You want to keep your save DCs high.
Metashadow Rods are also quite useful, though the good ones get expensive.
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