Spellcasting. Proecian casts spells as a 20th level spellcaster. Her spellcasting ability is Intelligence (spell save DC 27, +19 to hit). She has the following spells prepared:
Divine rejuvenation. Proecian heals 40 hit points at the start of her turn as long as she has 1 hit point.
Heart of the Forge. Proecian wears a ring on the middle finger of her right hand that serves as her artifact. This ruby-studded platinum ring grants any creature attuned to it the effects of the spell mage armor, and a +2 bonus to their Armor Class and Saving Throws. In addition, a creature other than Proecian gains immunity to fire damage, and resistance to radiant and necrotic damage. This artifact is sentient, with an Intelligence, Wisdom and Charisma score of 20, and it has two major beneficial traits, one minor beneficial trait, two minor negative traits, and one major negative trait (these traits only apply to creatures other than Proecian).
Inevitable return. When Proecian is reduced to 0 hit points, her soul is pulled beyond the Veil and she must spend time recovering before she can take physical form once more. She must wait for at least 1 year before she can make a new body to house her soul. If she was missing maximum hit points because of Hellfire when she is defeated, then she must wait for 10 years before she can make a new body instead. If she is reduced to 0 maximum hit points by Hellfire or some other means, Proecian is slain permanently.
Flames of purification. Creatures that have immunity to fire damage instead have resistance to fire damage dealt by Proecian.
Multi-action. Proecian either uses sphere of flame or teleport. Then she uses one of the following actions: Hellfire, Rain of Fire, or she casts one spell.
Hellfire. Up to three target creatures within 240 feet must make a DC 27 Dexterity saving throw. On a failed saving throw, a creature of Challenge Rating or level 9 or lower are instantly slain, while a creature of a higher Challenge Rating or Level instead reduce their maximum hit points by 75. In addition, a creature wearing non-magical armor that fails their saving throw has their armor reduced to ash, destroying it utterly. Any creature slain by hellfire is destroyed utterly, and all mundane and magical items of Rare or lower quality are destroyed with them. Proecian has her maximum hit points reduced by 35 for each creature she targets with Hellfire, regardless of whether they pass or fail their saving throw.
A creature who's maximum hit points are reduced my hellfire (including Proecian) cannot have these lost hit points restored through magical means. At the end of each week, a creature regains 1d10 lost maximum hit points.
Rain of Fire. Ranged spell attack: +19 to hit, 240ft range, up to 12 separate targets. Hit: 32 (4d10+10) fire damage.
Sphere of Flame. A globe of billowing blue fire forms up 20 feet from Proecian's current position. This globe is 20 feet thick, melts any earth that it comes in contact with to molten glass, and obscures all creatures that are within the flames or the center of the sphere. A creature within the globe itself is blinded. Proecian may see through this sphere without penalty.
A creature who starts their turn within the globe, or within 20 feet of it when it appears, or who moves within 20 feet of or into the wall for the first time on their turn, must make a DC 27 Constitution saving throw. On a failed save a creature is dealt 36 (8d8) fire damage, or half as much damage on a successful saving throw. This sphere lasts until the beginning of Proecian's next turn.
Teleport. Proecian teleports to an unoccupied space within 100 feet of her current location.
Proecian can take 3 legendary actions, choosing from the options below. Only one action can be used at a time and only at the end of another creature's turn. Proecian regains spent legendary actions at the start of her turn.
Cast a spell. Proecian casts a spell of 4th level or lower.
Shape molten glass. Proecian shapes an area of molten ground within 60 feet of her in one of the following modes. Each mode can only be used once between the beginning and end of Proecian's turns.
Burst shell (2 actions). Proecian bursts the Sphere of Flame that she is currently maintaining around her. All creatures within 60 feet the sphere must make a DC 27 Dexterity saving throw. On a failed saving throw a creature is dealt 27 (6d8) fire damage, and half as much damage on a successful save. All sand and dirt in this area becomes molten glass, and all unattended, non-magical items ignite as well.
A woman stands before you dressed in a golden robe. Her white hair flows down her shoulders unbound, ending in tips of black, and a single lock covers the left side of her face. The eye that you can see is violet, and dances with an orange, unearthly flicker, but it is cold and hard as she meets your gaze. Her skin is white and radiant, except at the tips of the fingers and the palm of her left hand, which are blackened and grey, as if covered in deeply embedded soot. Every once in a while, as the wind causes her hair to shift, you catch glimpses of her face, and can see that the same discoloration creeps up her neck and cheek in a ragged, uneven pattern, as if she were struck by a raging fire long, long ago. This human is Proecian, goddess of flame and innovation, and as she looks upon you, you can feel her taking the measure of your being.
Proecian is the wife of Bastin Caine, Lord of Light, and is counted among the most powerful human gods. With his help she managed to overcome the dragon Rhaul, and during the War of Judgement she slew her husband and a coven of ascendants, most of them before they could react to her interruption of their ritual. Her flames burn hot enough to turn mortal beings to ash in seconds, her command of the flames allows her to shape that which she destroys into almost any form she desires, and she is one of the very, very few beings who can channel Hellfire, the only substance that is known to be able to permanently kill immortal beings such as gods and dragons.
Those who would seek to battle her avatar are cautioned not to do so lightly, as they will be willingly standing against one of the more powerful ascendants to ever walk the world. Those who still decide to take this challenge upon themselves must prepare to potentially embrace oblivion if they should somehow manage to push the goddess too far.