Let me preface the following with this: Acererak is a genius asshole of incredible proportions. Not only does he have
enough intelligence (27 intelligence) to look at a
Beholder (17 intelligence), one of the most paranoid creatures in existence (meaning they can make incredibly elaborate plans to counter
virtually any situation,
no matter how unlikely), as something lesser than even the way a
human (10 intelligence) can see a
random crab (1 intelligence), his intelligence enters the realm of the supernatural, which grants him the natural ability to
completely predict an opponents actions.
This godly level of intelligence makes Acererak act with the utmost care. Acererak does not go and fight personally (not usually), but instead prefers to act through proxies/avatars/bodies. This is such a case for the
Demilich skull. This physical skull is likely from Acererak's
original body (stated tied to the dust from his old bones,
that were discarded from his physical form). Should be noted however, this skull
is not the true Acererak (his true phylactery and essence is beyond the Tomb of Horrors,
in the Tower of Conclusion within the Negative Energy Plane/Shadowfell):
–
Source
So what the Lich will be fighting here is a constructed avatar of Acererak, while his true self is watching, elsewhere. The demilich skull exists within 5 planes of existence (as a construct requiring a
weapon of +5 modifier), which means for it to receive damage physically, one needs to attack it across all these planes at the same time. Moreover, this skull is stated
immune to virtually all forms of magic, besides specific spells (for the abilities these spells would entail on Vs Battles-lingo: Memory Manipulation, Holy Manipulation, Death Manipulation and Power Nullification). But the skull, as an actual creature, possesses resistance to
these abilities. Not only does the skull resist all of these, but in fact,
as accepted on the hax layer evaluation thread, these are layered. Specifically, the Demilich skull resistance layers varies from
14 to 18 layers (a base resistance starts at 11, the construct has defenses of 25-29).
With all these being said, the construct skull harbors
a ghostly guardian. One who goes on to fight the skull has to contend against a ghost born from the bone-dust of Acererak. Anytime it is attacked, it gives the illusion of being dispersed, but instead the attacker is fueling the apparition, until it manifests fully as a ghost on the Ethereal Plane. The ghost can age a target (which the Lich doesn't care), make it flee in terror (while the Lich normally resists, its resistances seem to be baseline, while a typical ghost in D&D has at its lowest,
around 2 layers) or trap its soul (layered).
Note: Undead/souls in D&D predate reality, being
formed of entropy, with those like ghosts being
among the most powerful, with souls in particular
being completely immortal. Beings of similar existences to ghosts include
shadows
While the ghost attacks, the demilich skull has abilities of its own it can employ. As shown
here, the demilich has a fear aura that can nerf resistances, rays of death energy that renders souls (necrotic energy), as well as the ability to absorb and trap souls within its gems. All of these are layered, of course. When a demilich consumes a soul,
its body crumbles away and if the skull its destroyed, the soul also is destroyed.
So the Lich has to contend with this, while Acererak watches. He is in control of both the ghost and the demilich skull. He will want to get information about the Lich, its abilities, its equipment.
And fun fact, the Lich can't even hope to find him. You see, Acererak is adept as making his true form unfindable through divination effects,
and not even deities can find something which he wants hidden. Presumably (though not specified), not even beings of
this caliber can find him. So neither clairvoyance or cosmic awareness of the Lich can find Acererak. This gives him time to prepare, and
boi is he a prep master.
Acererak is capable of creating bodies from those of
dead gods and pilot them into combat, while absorbing their divinity to ascend. If he is stopped/destroyed, he immediately goes to his
true body and attempts to
ascend anyways (note: while he is noted to have the Eye of Vecna here, this is not noted on his profile nor is a normal occurrence), fighting to the end if it comes to it.
Acererak is capable of
killing and
sealing deities, and while these beings are noted as weak, for being capable of granting spells, they are of similar nature to demi-gods such as
Doresain or
Hercules. Gods in general have a
slew of
broken abilities
Note: these blogs are kinda outdated (like Acererak himself), but serve to showcase the kind of beings Acererak can contend with.
Adding to all this, Acererak
has access to a
Sphere of Annihilation that can erase the Lich and completely kill it (negging its immortality). Alongside it, Acererak has at his disposal an
Atropal, to which he can feed the Lich's soul/essence, again negging its immortality and regeneration.
All in all, Acererak (and even his lesser bodies) completely resist the Lich and his abilities save for one (The Enchiridion's plot manip), with a myriad ways he can kill it. And all of this ignores stuff like Far Realm spells (of which he undoubtedly has access to and/or can conjure through Wish).
I'm frankly seeing this as bordering on a stomp, depending on how the Plot Manipulation works. It's quite frankly a mismatch. Either Acererak stomps completely, Enchiridion or not or the Lich himself stomps with plot manip.
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All of the scans used here with their respective sources can be found
here