Archduke Abraham of Avernus

Old Abe was once a promising and powerful wizard in the school of Chronurgy, delving deep into the depths of the very fabric of reality and manipulating the weave to achieve whatever outcomes he desired. So deep was his maddening obsession with discovering these secrets that while trying to learn them all his mind withered and decayed as his warforged body began to rust. Eventually his mind was taken over by Eren the Prophet from the Cult of Bhaal for a short time before being freed by "Fred" of Waterbet. Old Abe learned a lot in his time of possession, and the last straw snapped in his mind unleashing centuries of contained passion and sending Old Abe into a spiralling frenzy of uncontrollable fixation on the domination and eradication of evil from all planes, so that his mind never be compromised again. 

One day while toiling away in his lab Abe went a bit stir-crazy and decided that it would be a great idea for him to strip naked and then Wish to swap bodies and equipment with the Archduke of Avernus, crippling them by giving Zariel the weak and feeble body of Old Abe and granting Old Abe the powers of Archduke Zariel of Avernus. With her grand works achieved of an immortal and powerful body, Old Abe has absconded with an Amulet of Proof against Detection and Location to hide somewhere in the planes and test the limits of her newfound power. 

With a new, ageless and more powerful female body that wont age or remain dead if it dies outside of Avernus, a place where Old Abe refuses to go, Old Abe spends her new forever tricking Evil creatures into Wishes that ultimately backfire against them as Old Abe can peer into every possible consequences of any wish and pull forth the one he desires. In payment for this crooked wish they find themselves bound by a powerful 9th level Geas spell that compels their servitude. She wickedly delights in this enslavement of Evil thinking that this is serving Good but the results are questionable. Old Abe maintains an active trading relationship with the Evil Efreeti in Pyroclast who think they are selling Slaves to join the armies of Avernus but who ultimately end up serving just Old Abe, since Old Abe can create simulacrums that wish for as much gold as needed, Old Abe has greatly enriched the Evil Efreeti while trying to eradicate this Evil. As such however, the Efreeti are very protective of this important revenue stream. It is not uncommon to see Old Abe in the presence of one or more Noble Efreeti. 

When encountered, Old Abe will almost always be under the effects of an Alter Self to disguise his appearance as a younger anthropomorphised idealised version of himself as a powerful medium humanoid human wizard. If this is dispelled through the use of Dispel Magic then his true, horrifying demon form will be revealed, at which time Old Abe will normally turn invisible and fly away as fast as possible. He takes great pride in travelling anonymously hidden by his Amulet of Proof against Detection and Location and will go to great lengths to protect his identity, as while he desires to use the powers of Archduke Zariel of Avernus, he does not want to be seen as Zariel, when he desires to be seen as a more powerful form he will typically create the illusion of a Solar but attack with a fiery Flail and the Mighty Matalotok, taken from Archduke Zariel of Avernus who has probably sent all the forces of Avernus to try and locate it, so which these weapons are kept hidden until absolutely needed, with Abe instead preferring to manipulate the flow of time in desirable ways so as to avoid or end combat as soon as possible, with an array of powerful magic available to him.

Old Abe's Lair

Old Abe makes her lair in a permanent Mighty Fortress on the House of Knowledge (Celestial Plane) that is heavily protected against unwanted visitors. There is a Forbiddance spell on each of the Mighty Fortresses Four Mighty Turrets cast in a 2x2 grid of 120 square feet squares. Four Private Sanctum spells protect a 2x2 rid of 60 square feet squares that share a boundary with the Mighty Fortress.There is a 3x3 grid of 40 square feet squares encompassing the bounds of the Mighty Fortress within Guards and Wards spells. Inside the Mighty Fortress is a Temple of the Gods devoted to Oghma who allows Old Abe to peacefully reside on his plane, Old Abe often hosts avatars of Oghma inside this Temple. Several Druid Grove's surround the Mighty Fortress.

Druid Grove: Cast in 30-foot cubes surrounding the mighty fortress. There is a password known by Old Abe and Abe Jr. that, when spoken aloud, makes the speaker immune to these effects. The entire warded area radiates magic. A dispel magic cast on the area, if successful, removes only one of the following effects, not the entire area. That spell’s caster chooses which effect to end. Only when all its effects are gone is this spell dispelled.
Solid Fog. You can fill any number of 5-foot squares on the ground with thick fog, making them heavily obscured. The fog reaches 10 feet high. In addition, every foot of movement through the fog costs 2 extra feet. To a creature immune to this effect, the fog obscures nothing and looks like soft mist, with motes of green light floating in the air.
Grasping Undergrowth. You can fill any number of 5-foot squares on the ground that aren’t filled with fog with grasping weeds and vines, as if they were affected by an entangle spell. To a creature immune to this effect, the weeds and vines feel soft and reshape themselves to serve as temporary seats or beds.
Grove Guardians. You can animate up to four trees in the area, causing them to uproot themselves from the ground. These trees have the same statistics as an awakened tree, which appears in the Monster Manual, except they can’t speak, and their bark is covered with druidic symbols. If any creature not immune to this effect enters the warded area, the grove guardians fight until they have driven off or slain the intruders. The grove guardians also obey your spoken commands (no action required by you) that you issue while in the area. If you don’t give them commands and no intruders are present, the grove guardians do nothing. The grove guardians can’t leave the warded area. When the spell ends, the magic animating them disappears, and the trees take root again if possible.
Additional Spell Effect. You can place your choice of one of the following magical effects within the warded area:
 - A constant gust of wind in two locations of your choice
 - Spike growth in one location of your choice
 - Wind wall in two locations of your choice
To a creature immune to this effect, the winds are a fragrant, gentle breeze, and the area of spike growth is harmless.

Forbiddance: 2x2 grid of 120 feet-squared squares centred on the turrets of the mighty fortress and meeting perpendicular to the axis of the mighty fortress and radiating out 60 feet from the mighty fortress up to the starting boundary of the druid grove's. Creatures can't teleport into the area or use portals, such as those created by the gate spell, to enter the area. The spell proofs the area against planar travel, and therefore prevents creatures from accessing the area by way of the Astral Plane, Ethereal Plane, Feywild, Shadowfell, or the plane shift spell. In addition, the spell damages celestials, elementals, fey, fiends, and undead. When a chosen creature enters the spell's area for the first time on a turn or starts its turn there, the creature takes 28 (5d10) radiant damage. There is a password known only by Old Abe and Abe Jr. 

Private Sanctum: protects a 2x2 rid of 60 square feet squares that share a boundary with the Mighty Fortress. It has the following properties:
 - Sound can't pass through the barrier at the edge of the warded area.
 - The barrier of the warded area appears dark and foggy, preventing vision (including darkvision) through it.
 - Sensors created by divination spells can't appear inside the protected area or pass through the barrier at its perimeter.
 - Creatures in the area can't be targeted by divination spells.
 - Nothing can teleport into or out of the warded area.
 - Planar travel is blocked within the warded area.

Guards and Wards: 3x3 grid of 40 square feet squares encompassing the boundaries of the mighty fortress. Old Abe and Abe Jr. are immune to these effects. There is a password known by Old Abe and Abe Jr. that, when spoken aloud, makes the speaker immune to these effects.
Corridors. Fog fills all the warded corridors, making them heavily obscured. In addition, at each intersection or branching passage offering a choice of direction, there is a 50 percent chance that a creature other than you will believe it is going in the opposite direction from the one it chooses.
Doors. All doors in the warded area are magically locked, as if sealed by an arcane lock spell. In addition, you can cover up to ten doors with an illusion (equivalent to the illusory object function of the minor illusion spell) to make them appear as plain sections of wall.
Stairs. Webs fill all stairs in the warded area from top to bottom, as the web spell. These strands regrow in 10 minutes if they are burned or torn away while guards and wards lasts.
Other Spell Effect. You can place your choice of one of the following magical effects within the warded area of the stronghold.
 - Place dancing lights in four corridors. You can designate a simple program that the lights repeat as long as guards and wards lasts.
 - Place magic mouth in two locations.
 - Place stinking cloud in two locations. The vapors appear in the places you designate; they return within 10 minutes if dispersed by wind while guards and wards lasts.
 - Place a constant gust of wind in one corridor or room.
 - Place a suggestion in one location. You select an area of up to 5 feet square, and any creature that enters or passes through the area receives the suggestion mentally.
The whole warded area radiates magic. A dispel magic cast on a specific effect, if successful, removes only that effect.

Temple of the Gods: One 30-foot cube in the north-east corner of the mighty fortress. The walls are polished white marble with gold intricately veined through the marbling, there is one large solid platinum door on the south side and 5 large intricate stained glass windows, 2 on the east and west sides and 1 very large window on the north side roof. Only Old Abe and Abe Jr. may open or close the door. The area is brightly illuminated and is warded against (opposes): celestials, elementals, fey and undead. If a creature of the chosen type attempts to enter the temple, that creature must make a Charisma saving throw. On a failed save, it can’t enter the temple for 24 hours. Even if the creature can enter the temple, the magic there hinders it; whenever it makes an attack roll, an ability check, or a saving throw inside the temple, it must roll a (d4) and subtract the number rolled from the d20 roll.
In addition, the sensors created by divination spells can’t appear inside the temple, and creatures within can’t be targeted by divination spells.
Finally, whenever any creature in the temple regains hit points from a spell of 1st level or higher, the creature regains additional hit points equal to your Wisdom modifier (minimum 1 hit point). (+1)
The temple is made from opaque magical force that extends into the Ethereal Plane, thus blocking ethereal travel into the temple’s interior. Nothing can physically pass through the temple’s exterior. It can’t be dispelled by dispel magic, and antimagic field has no effect on it. A disintegrate spell destroys the temple instantly.

Lair Actions. On initiative count 20 (losing initiative ties), Old Abe can take a lair action to cause one of the following effects; she can’t use the same effect two rounds in a row:
 - Old Abe casts major image four times at its lowest level, targeting different areas with the spell. Old Abe creates images of intruders and their loved ones rapidly and painfully aging and screaming. Old Abe doesn’t need to concentrate on the spells, which end on initiative count 20 of the next round. Each creature that can see these illusions must succeed on a DC 15 Wisdom saving throw or become frightened of the illusion for 1 minute. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
 - Old Abe casts her innate fireball spell.

Regional Effects. The region containing Old Abe’s lair is warped by her magic, which creates one or more of the following effects:
 - The area within 9 miles of the lair is filled with an ominous sense of déjà-vu, events seem to happen the same way again and again. If a creature makes the same ability check twice or more in 24 hours for any reason then for each time after the first dice roll there is a 20% chance of ignoring the die roll and repeating the earlier result.
 - Once every 60 feet within 1 mile of the lair, 10-foot-high cracks in space-time appear from nothing and occupy a 5ft square, randomly teleporting the creature 33 (6d10) feet away in a random direction. Any creature or object that touches the fissure takes 7 (2d6) force damage, though it can take this damage no more than once per round.
 - The area within 2 miles, but no closer than 500 feet, of the lair is filled with smoke, which causes the area to be heavily obscured. The smoke can’t be cleared away.

If Old Abe dies, these effects fade over the course of 1d10 days.