Short version:

Spells can be crafted with:

• elements (elemental sigils)
• sigils (more advanced sigils for more advanced mechanics)
• demonic will (in all forms (i.e. ethereal (aura), solid (crystals), stored in tartaric gems...)
• life essence (liquid, in form of blood orbs (which are equivalent to some part of their capacity), damage dealt while crafting...))
• mobs (containing them on an alchemy array)

Spells can be crafted in:

• a spell focus (crafting grid that can also store will for example)
• alchemy arrays (or an array of alchemy arrays in a neat formation)
• with rituals (or containing the effect of a ritual in a spell)
• with the previously suggested "slotted shield" (Sigils adding effects to shield) i.e. binding a spell to a shield (or weapon), spell focus spells could be slotted in a shield for example

Spells could be affected by:

• the "healthiness" of your soul network (filled to the brink soul network could overmodulate your spells, an empty soul network could kill you or affect you with debuffs, make the spell weaker etc. (the spell could use a combination of will and LP))
• the "healthiness" of the chunk demonic will aura (possibly naturally occuring will, similar to aforementioned soul network healthiness)
• same for tartaric gems or needed physical ingredients (will crystals)
• just modified by the type of will in the chunk (raw will makes it generally more potent, for example, corrosive will has negative side effects unless it is corrosive, some sort of magic alignment chart would be helpful (i.e. order vs chaos vs good vs evil for will/essence, possibly including types)
• potion effects
• specific baubles/items to affect spells
• living armor upgrades
• sentient armor (could also marginally disperse ethereal will into the chunk)
• rituals
• etc...

Useful items:

• ingredient pouch! (to hold will crystals, tartaric gems (with a special slot for the one you want to use for your equipment), ethereal will etc) [doesn't have to be a pouch but related]
• Bound Tools! (Spells bound to tools, eh?)

Elaboration follows discussion.