The player is the boss fight

[Belthuzar]

My dm has proposed an idea to me. She wants me to make a character that will, later on, betray the party and escape to do something evil in the background. She has given me 2 stipulations.
1 I have to make use of the Shadow School Spells (Shadow Conjuration, Shadow Enchantment etc.)
2 I have to play a human cleric with the rune/legislation domain. He's supposed to have a law mage/black knight vibe
In return, she will allow me to costume make a god suit the build and add it to the lore. I excepted of course because it sounded dramatic. The game currently has almost no magic items and we aren't getting many anytime soon. We are currently using the automatic progression bonus system and the elephant in the room feat tax system to offset this. (https://michaeliantorno.com/feat-taxes-in-pathfinder/) There are a few things that need to happen to make this work. 1 I have to be able to basically hide my *power level* and 2 when I go off I'm probably going to have to fight 7 on level characters who are not min-maxed but competently built including an archer paladin anywhere from levels 9-13. Either than that anything that is 1st party pathfinder is fair game.

[Power]

Hi! This kind of thread is what the Min/Max It! Subforum is for. The sticky in particular is a useful guideline to posting threads of this nature.

From what I can gather:

• There are no magic items available.
• Betrayal will occur at levels 9-13, and character has to be able to pose a major threat to a party of 7, including an archer paladin.
• All 1st party content is legal.
• The variant rules systems automatic bonus progression and reduced feat taxes are being applied.
• Character must be a Human race lawful Cleric who makes significant use of shadow spells, uses the Rune (Legislation) subdomain, and has a "black knight" vibe.
• You may make your own deity.

My questions:

• What kind of availability is there for nonmagical items? Especially when it comes to expensive spell reagents, alchemical items, mithral armor, etc. What kind of wealth can you expect to have?
• What is your point buy?
• Is your character meant to be outright evil alignment (considering the presence of a Paladin)? I assume so, but that will also work oddly with Detect Evil and how your character is supposed to be part of the party.
• Is your deity meant to be evil?
• Is your character meant to make melee attacks?
• Under what circumstances is your character meant to encounter the party? Do they get the drop on your character, do you ambush them, do you have time to prepare? Do you get to pick your battlefield?
• Am I correct in assuming that the boss fight will just consist of your character and whatever preparations you have?
• What else do you know about the party? Class, race, build?