Souls and XP

Magical item crafting relays an XP cost, which represents the amount of essence the crafter infuses the object with. In DnDMFA the most important way is using the souls of slain creatures/beings.

Souls
As is well known, every thinking being, creature, or well, a monster (that is, one with an INT greater than three) has a soul. The soul contains energy that can be harnessed, crafting of magic items is one of the ways to do so, by embedding the soul inside the object in place, as long as the XP requirements for the item's creation are satisfied, or exceeded. The soul has to be used whole in the process - "soul fractions" do not really exist, and we can assume that once embedded, a soul can't be taken out of a magic item by common means, leaving partially depleted souls for someone else to worry about

To capture a soul, one needs the Soulstealer feat, available to angels, demons and cubi (and dragons ofc.) or the appropriate spell, like Soul Trap.

The potential of being and creature souls is the XP the character has collected (as evidenced by its CL) + 25*(its Con score)

Example: Ulan, a freshly levelled up canine warrior makes his eternal life's last mistake by going to test his newfound strength by challenging Baszat, a local demon overlord. Baszat kills him and harvests his essence into a jar. Ulan was a level 2 warrior with a Con score of 15, therefore, his soul can be used for crafting an item of an XP cost up to 1375 XP.

For monsters, the collectable amount is 25*(Con Score) + 1000*(Monster's hit dice)

Creating an intelligent item
If the amount of soul energy deposited into an item in XP exceeds the cost of the item, and if the bearer of the soul had sufficiently high mental attributes, the item might develop sentience.

This works as following:

If the XP usage doesn't reduce the character's soul below level 1, Roll will save of the soul's owner against DC 15+2x(the level loss associated with the XP drain used for the item's properties). If the character passes the save, it's soul has stayed uncorrupted (in the bad/missing data sense) enough for the item to manifest the soul owner's characteristics.

The item gets mental stats as the original owner of the soul did, with its powers being decided by its highest mental stat meeting the highest stat mentioned in the chart of DMG page 269. If the item doesn't meet any of the prerequisites, it still can communicate through empathy and has a vision and hearing range of 30 ft., but it lacks any useful powers. The allignment matches the original soul-owner.

The cost of enabling a chosen power is one third of the listed price in DMG page 269 and 270

Example: Baszat decides to make a Ring of Wizardry(I) from the soul of the aforementioned rogue. As such, he uses up 800 XP, causing the drain of one level. The rogue's mental stats are Int 11, Wis 7 and Cha 10 He rolls the save of 1d20-2 against a DC of 17, and gets a 19 on the roll, meeting the DC. His spirit is preserved reasonably intact despite the drain, but since his highest mental attribute is 11, which is below the threshold, the item gets no powers. He however can communicate through empathy, and has a hearing and vision range of 30 ft.

Subduing an intelligent item
This all sounds rather simple, however, it isn't the end - if the item has preserved some consciousness, it probably now wants to do you harm and sabotage your every action , not even speaking about the fact that your allignments are most likely differing, which together will cause you level drain as prescribed in the DMG, nevermind other problems by the item misbehaving ...

The subduing ritual uses up one casting of Geas or Quest spell, and money based on the highest mental score of the item, (In the case of highest Wisdom using potent ingredients to force the enchantment onto the reluctant entity, in the case of highest Charisma using potent wards to prevent the item from subjugating you instead, and in the case of highest Int, hiring a lawyer to draft a suitable geas text)

The cost of this enchantment is 160gp x (the highest ability score of the item) x (highest ability score of the item)

Example: Now, Baszat accidentally obtained an item that doesn't like him the least, and will attempt to grant negative levels to him or his minions if they use it. He knows that Ulan's highest mental characteristics was 11, therefore he'll need to spend 8800gp (as well as one casting of Geas/Quest) on the ritual to prevent Ulan's angry spirit to do him any harm. In the end, he calculates the total cost to be 19360 - just barely better than buying the item off-market, so he sighs and carries on with it, ending up with a grumpy Ring of Wizardry(I) who however is unable to cause him or his minions level loss, give him false information, or knowingly aid those who seek him harm.