All Harrowed were once living creatures. To create a Harrowed, you must start with a ‘template’ character. You can either use a previous character who met a painful end, or roll up a first-level starting character, work with the DM to determine how they died, and go from there.
You retain all the racial traits granted by your previous race, including size, creature type, speed(s), skill and tool proficiencies, and any languages you used to speak. You also retain your personality, alignment, and memories of your past life, along with any class levels you have acquired.

Your creature type is determined by what you used to be, as stated above. You are also considered an Undead by any prerequisite or effect that requires you to be Undead.

Your Constitution increases by 2. This is in addition to any increases your previous race granted you, and can raise your Constitution score above 20.

As mentioned above, you have a Quori inside of you, an evil spirit that you share your body with. It has a Lawful Evil alignment and Intelligence, Wisdom, and Charisma scores of its own. The DM sets the Quori’s abilities or determines them randomly (roll 4d6 for each score, ignoring the lowest and totaling the rest). The Quori can see and hear using your senses regardless of whether it’s in control. It cannot speak to you directly, but it can communicate to a limited extent through the dreams you experience during a Long Rest, as per the Dream spell.

The Quori has an agenda of its own, and as a Lawful Evil creature, that agenda likely conflicts with your own (unless you also happen to be Lawful Evil). For the most part, the Quori simply sits back and lets the Harrowed go about his daily unlife. However, if the Quori sees that you’re about to pass up an opportunity to do something evil or advance its agenda without endangering yourself excessively, it may decide to try and wrest control from you. If it's strong enough, it can take control of your body for a few hours. And if it gets too strong, it can take over permanently.

This constant struggle for control is represented by your Dominion modifier. When you first rise from the grave, your Dominion modifier starts at 0, meaning that you and the Quori are on roughly even footing. When the Quori attempts to take control, you must make a Charisma saving throw (DC 15 + your Quori’s Charisma modifier + your Dominion modifier). If you succeed, you retain control, and if you succeed by more than 10, you subtract 1 from your Dominion modifier. If you fail, the Quori takes over for 1d6 (in-game) hours, and if you failed by more than 10, the Quori adds 1 to your Dominion modifier. No matter how many times you win or lose this struggle, your Dominion modifier can never go below –10 or above 10. 

When in control, the Quori uses your body, stats, and abilities as if they were its own, and you cannot see or hear anything it does. In essence, your character becomes an NPC that is controlled by the DM for as long as the Quori is in control, and a hostile NPC at that. 

Any time you are rendered unconscious when your Dominion modifier is at or above 5, the Quori may attempt to take permanent control, forcing you to make a Charisma saving throw (DC 15 + your Quori’s Charisma modifier + your Dominion modifier). If you fail, your old personality, alignment, and identity are permanently suppressed by the Quori, and your character becomes a NPC no longer under your control (and possibly a new villain with a very familiar face to boot).

As a Harrowed, you are sustained by your Quori’s unholy energies. You are completely immune to the negative effects of disease, starvation, dehydration, or suffocation/drowning. You are also immune to Poison Damage, and resistant to Necrotic Damage and nonmagical bludgeoning, piercing, or slashing damage. In addition, you do not age over time, and completely ignore any magical effects that increase or reduce your age. 

You are immune to Massive Damage, and automatically become Stable when you fall to 0 hit points as if your attacker knocked you out with a melee weapon, regardless of the intent of the attacker or nature of the attack. In addition, any damage you suffer while at 0 hit points only incurs a Death Saving Throw Failure if that damage targets your head. Note that damage that affects the entire body (i.e. being dumped in acid or lit on fire) affects the head too, and can thus kill you just as easily as damage that directly targets your head.

As a Harrowed, you do require sleep, or at least something approaching it. You can finish a Long Rest in 2 hours, but you must spend all of those two hours in an unconscious state, and you can’t push yourself to stay awake as well as a mortal can, on account of the Quori inside of you. You can try and fight the Quori to stay awake, but you must make a Charisma saving throw (DC 15 + your Quori’s Charisma modifier + your Dominion modifier (see below)); if you fail, you instantly fall unconscious on the spot. Fighting the Quori like this is also incredibly taxing; if you succeed, you must make a Constitution saving throw (DC 10 + 2 per hour spent without sleep) or gain one level of exhaustion. 

Fortunately, completing a Long Rest instantly reduces your exhaustion level to 0. You can also complete a Short Rest in just 10 minutes.

Strangely, you do still need to eat… at least if you want to recover Hit Points. You only regain spent Hit Points or Hit Dice during a Short or Long Rest if you’ve eaten at least a pound of meat in the last 24 hours. Fortunately, the quality of the meat does not matter; even a pound’s worth of demon meat that’s been rotting for centuries will suffice, though it probably won’t taste all that great.
You also heal far faster than a mortal; when you spend a hit die during a Short Rest, instead of rolling the die, you just take the die’s maximum possible result (8 for a d8, 6 for a d6, etc).