This ritual is called Mage Armor by many, but barbarian tribes and monastic orders are known to make use of it as well. It can manifest as iron-hard skin that deflects blows, an ephemeral suit of armor fashioned from one of your Attuned Elements, reflexes honed by your connection to otherworldly forces, or any other cosmetic and narratively appropriate effect.
When you gain this feat, choose either CON or your Primary Spellcasting Attribute. You cannot change the chosen attribute without Retraining this Feat.
When you cast this ritual, you apply a protective Infusion to yourself that lasts until you finish a Long Rest or dismiss it with a Minor Action.
While the Mystic Armor is active and you are not wearing any armor, you gain a Bonus to your Defense saving throw equal to the chosen attribute's Attribute Modifier, and Armor DR equal to your CON modifier multiplied by your Tier.
While this functions like a true Infusion, it is more flexible in how you can maintain it. You can either use the Sustain action to maintain it like a normal Infusion (which functions normally with the Infusion Focus ability), or have it siphon mana directly from you to sustain the effect; this reduces your Mana Capacity by -2 (to a minimum of 0) but removes the need for any use of the Sustain action to keep it going.