The Dark Guardian

The Dark Guardian is an artificial skeleton monster assembled by a nearly omnipotent cube-like entity named simply "The Registry." It's sole purpose in life is to assist the Registry in examining various different kinds of dimensional spaces within the common Dimensional Hierarchy, reporting back the intricate details of that certain dimensional space to the Registry as it is too busy keeping record of all the dimensional spaces within the Dimensional Barrier. Although it isn't too good at that job, often times it is too busy napping on the job. While it isn't napping, it almost never takes it's job seriously, which is why the Registry often has to inject the Dark Guardian with it's core liquids. This results in the artificial creation having it's mind completely overwritten into actually caring about it's job, even giving him the ability of near-omnipresence within the Dimensional Barrier to report the hidden details in each dimensional space and plenty others. While there are far more responsibilities that the Registry must uphold, it usually handles them by itself since it doesn't trust that any of it's employees can get those jobs done right.

Backstory
The Dark Guardian's backstory is actually quite simple, it is simply an artificial being created by the Registry taking a clump of it's core liquids and shaping it after a certain skeleton from a certain Universe. Though it must have been a malfunction creation, as it is the laziest of all of the Registry's self-made employees. The only reason the Registry hasn't exterminated it for poor performance is because it believes that taking the life from your own living creation while it hasn't done anything inherently wrong is unforgiveable... but mainly because destroying it would result in a loss of it's core fluids.

Appearance
The Dark Guardian looks much like classic Sans, only with a few outfit changes; it's jacket is entirely white as well as a bit longer, it has an orange scarf that hangs out in six different locations, it's wearing a white hood, it wears black and orange boots, and it wears mainly black clothes underneath it's jacket. While injected with core fluids; it's eyes turn orange, it's head becomes shadowed by it's hood, it's scarves begin to gain sentience floating around like several wiggly snakes, and it emits orange fog as well as white pixels.

Normal Form
The Dark Guardian's standard form, completely unenhanced by the Registry yet.

Teleportation/Universal Travel
Self-explanatory, the Dark Guardian is able to travel across various Universes inside of the Multiverse that it was inserted into.

Bone and Gaster Blaster Magic
The Dark Guardian can use bones and Gaster Blasters at it's will. In this form, it's attack potency using these is only about ten times better than classic Sans', meaning they won't do much good against any threats that are somehow able to see it.

Absolute Cloaking
This applies to both forms, the Dark Guardian cannot be seen by anything inside of a Hyperverse or lower. In fact, it just doesn't even EXIST in the eyes of anything that is unaware of it. The only thing that can see it are of course, the Registry itself, as well as beings beyond a Hyperverse that somehow know of it's existence and the Outsiders, but we don't talk about them in full detail...

The Dark Hammer
One of the most defining features of the Dark Guardian is it's giant hammer. While not injected with core fluids, the hammer can only do as much as topple large buildings and cause earthquakes of 6.5 magnitude and lower, depends on how hard the hammer is swung. Though it barely does much damage in normal form, it will always remain totally indestructible throughout all of the Dark Guardian's forms.

Injected Form
The Dark Guardian's enhanced form, only occurs when the Registry injects it with it's core liquids. This form is only temporary, meaning that once it's been long enough, it will become like a lifeless ragdoll and must be recharged at the Registry to regain it's consciousness.

Omega-Enhanced Bone and Gaster Blaster Magic
In this form, the Dark Guardian's bone and Gaster Blaster magic is now enhanced to the point that even a single bone can wipe out two or three Multiverses, one Gaster Blaster would be enough to severely damage a Xenoverse, and all of the efforts of this ability alone put together could quite potentially destroy an entire Omniverse.

Omega-Enhanced Teleportation/Outerversal Travel
In this form, the Dark Guardian's teleportation is also enhanced by an incredible margin, allowing it to pretty much teleport anywhere within the Dimensional Barrier.

Mind Wipe
Should anything know about the Registry's operation, the Dark Guardian will just latch it's many scarves onto the target's head, draining their mind of any trace of the Dark Guardian or the Registry. Once done, the Dark Guardian will simply cease to exist in the eyes of it's target.

The True Dark Hammer
This is the Dark Guardian's most powerful attack in general as it is capable of creating Metaverse-shattering earthquakes, Xenoverse-annihilating explosions of Hyperversal proportions, and if targeting a single object can destroy an entire Omniverse or TWO. Apart from a ridiculous increase in strength, the hammer also gains an orange glow around it, occasionally even flashing something inside of the glow.

Trivia

 * While the Dark Guardian has a devastating array of abilities in it's Injected form, it doesn't really have any need to cause any dimensional destruction as it's sole objective is to merely examine the dimensions for any hidden details.
 * The only times that would really call for any dimensional mayhem is if something horribly wrong happens in that certain dimension.
 * To no surprise, the Dark Guardian in it's normal form is the least productive of all of the Registry's employees.
 * The Dark Guardian received it's name from the Registry itself to try and make it feel appreciated despite it's lack of productivity.
 * ...Too bad it doesn't really give two shits.
 * As stated before, the Dark Guardian does not originate from any Universe. It was simply shaped after Sans to add the sense of familiarity to it.