You can speak, read, and write Common and Undead.

(Through the use of proficiency with the language Undead you can communicate with Undead Creatures)

Instead of sleeping, you normally enter an inactive state for 4 hours each day. You do not dream in this state; you are fully aware of your surroundings and notice approaching enemies and other events as normal. However, on rare occasions (If the DM decides) you may fall into a deep dreamlike state, reliving past events of your life.

You gain:

Resistance to: Piercing, Necrotic

Immunity to: Poison, Poisoned

Vulnerability to: Bludgeoning, Radiant

You cannot swim. If you enter a liquid, you will immediately sink to the bottom unless the DM says otherwise or the liquid is smaller than your character. You cannot suffocate or drown.

Your body is not as strong as it once was. If the player receives more than their Character Level x (1 + their Constitution Modifier) worth of damage that they are Vulnerable to, one of their bones will Shatter. Shatter is an effect that can only be experienced by an Ancient Skeleton. 

If you are afflicted with Shatter, roll 1d6. The result of the dice roll will decide which bone has been destroyed unless the DM mentions otherwise.

• You hear a cracking before your vision goes dark and you must rely on your foresight to predict your surroundings. If you roll a 1 your skull is destroyed. Your range of vision is decreased to 10 ft of blindsight and all attacks against you have advantage.

• Your weapon falls to the floor. If you roll a 2-3 your arm is destroyed. You drop your weapon and receive a penalty of -5 to attack rolls. Two handed weapons deal -5 damage if wielded in this state. These penalties stack twice and can be alleviated by having an Arm Attachment. If you have no arms left you cannot perform actions that require arms. 

• You topple over as your leg crumples under you. If you roll a 4-5 your leg is destroyed. You fall prone and receive a movement speed penalty of 10ft.

• You lift off of the ground as your torso crumples sending the rest of you flying. If you roll a 6 your torso is destroyed. You are thrown in a 30ft line in the opposing direction of where you were hit, you can be caught at by an ally within a 5ft radius of the line you are being thrown down by performing a DC 15 Dexterity check as a Reaction. When you land you fall prone, unless caught by an ally. Until you recover from this state you are classified as Small and receive an AC penalty of -2.

You can use a Bonus Action to restore 1d8 + your Constitution bonus of health, this costs 1 charge. Or you may use a Bonus Action to recover from the Shatter effect, this costs 3 charges.

As an additional effect you may use an Action and Bonus Action to consume 5 charges and Temporarily gain an Attachment from a nearby corpse. You may obtain multiple new Attachments up to a maximum of 2. This will give you an effect depending on what the Attachment may be (or what the DM has decided). This Attachment is unstable so will become destroyed after a Long Rest.

When you obtain this new Attachment, the Shatter die Temporarily changes to 1d8 (1d10 at level 15 onwards) and a 7 and 8 will destroy it. For 2 Attachments a 7 corresponds to the first one added and an 8 for the latter. For 3 Attachments the third is 9 and 10 and for 4 Attachments, 9 for the third and 10 for the 4th. 

You can use Bone Harvest a number of times equal to your Proficiency Bonus + half of your Constitution Modifier (Rounded up). On a Short Rest you regain half  of your charges of this ability (Rounded up), on a Long Rest you regain all charges of this ability.

As you grow your soul regains its forgotten strength. At level 8 the maximum number of Attachments you can gain increases to 3, at level 15 it increases again to 4. At level 20 you remember how to maintain 2 Attachments at a time, ignoring the 'Until Long Rest' requirement and being destroyed only when Shattered or removed by choice.

Attachment Table (To be used as a basis):

Wing/s: You extend your senses into your new limb and cold blue flames filter into the gaps creating a mystical mesh. Gain 10ft of Flying Speed. If you have 2 wings as attachments, your granted Flying Speed increases to 30ft.

Arm/s: You crush the side of your torso as you ram an additional arm into your ribs. Take 2d4 bludgeoning damage and gain an additional arm. You can use this arm to perform a Special Action this can be: An Unarmed Strike, A Grapple, A Shove, Using an object or as a replacement for a Shattered arm. If you have 2 additional arms, you may also perform an Extra Attack with a secondary weapon and you are granted 10ft of Climbing Speed.

Exoskeleton: You tear the exoskeleton from a nearby corpse (given that they have one) and repurpose it for defensive purposes. You may attach an exoskeleton as a shield, fusing it with one of your arms. This provides an additional 2 AC and provides Damage Resistances (Not including Psychic, Bludgeoning and Radiant) of the creature for as long as the Exoskeleton is in use. (You may only use 1 exoskeleton at a time)

When the exoskeleton is destroyed the attached arm is also destroyed. The player is told which damage resistances the exoskeleton provides if they have seen the exoskeleton attacked with a resisted damage type.