Very Rare Spellcrafting

At the end of the Very Rare Spellcrafting casting time, the 80 gp Special Ingredients are consumed and the spell slot of the Specific Spell is expended, then you may make a DC 13 arcana check. If you succeed this check you make 1 day of progress in crafting your Very Rare Magical Item. On a failed check you make no progress. Your Very Rare Magical Item is completed when you reach 125 days of progress. 

At Higher Levels. You make an additional day of progress for each spell slot level used higher than 5th.

*You must have the Specific Spell known, prepared, or otherwise available and expend a spell slot of its level in addition to casting the Very Rare Spellcrafting spell. This remains true even if you cast Very Rare Spellcrafting as a ritual spell. This spell's school of magic is the same as the requisite Specific Spell's school. In some cases, a Very Rare Spellcrafting might list more than one Specific Spell, in these cases each Specific Spell must be available, but only the higher of these spells slot is expended.

**Ritual Cast Time: When this spell is cast as a Ritual Spell, the casting time is 8 hours. 

Sidebar: This spell allows spellcasters to create a specific magic item. This spell has as many versions as there are types of Very Rare Magic Items. One Wizard might know Very Rare Spellcrafting - Amulet of the Planes, and another might know Very Rare Spellcrafting  - Ring of Regeneration. Some item versions of this spell may have slightly higher or lower Base Material Cost, or might require more or fewer Days of progress to complete. A Very Rare Spellcrafting - Oil of Sharpness, for example, would have a Base Materials cost of only 10,000 gp, and require only 62 days of progress to be completed. As a general guiding principle: the special item cost, assuming only successful arcana checks and cast at the lowest level, should be roughly 25% of the item's market value, and the base materials should be close to 50% of the item's market value, and should together total close to 75% market value. This leaves 25% cost savings if all arcana checks are successful, and runs higher than the market cost if more than half these checks fail.

(This is an original spell/magic item crafting system)