Domain of Life and Grave

Clerics of Life and the Grave respect the dead, guide the dying onwards to the afterlife and pay homage to their achievements in the mortal world. They ease the suffering of creatures whose time has come to pass on. Their knowledge and understanding of life and death gives them the power to stave off a creature's death, though they refuse to use such magic to extend a creature's lifespan past its mortal limits. 

Clerics of this domain might pay fealty to a duality of deitys within these different domains, so long as these deities do not have polar-opposite interpretations of how life and death work within the natural order. For example, a cleric might worship Kelemvor, lord of the Grave who is an enemy of undeath - it being considered an affront to the natural order, whilst also paying worship to Lathander, a deity respecting renewal and birth - also opposing the taint of undeath. 

Clerics of this domain are devoted to maintaining a balance betweeen sustaining life as it exists in the mortal realm, whilst acknowledging that sometimes it is time for a creature to pass on from this world and be welcomed into the judgement of their patron of death. 

As deities of Life and Death can both often be opposed to Undeath due to its diversion from the natural order of things, Clerics of this domain will be believe similarly and be less inclined to use necromantic powers which would rob spirits of creatures being able to move on following their deaths. That being said, the use of powers such as revitilization and resurection of the recently departed are not necessarily anathema to clerics of this domain. Ultimately, those of this domain must use their own judgement as to whether or not it is right to preserve the life of those recently departed, and to bring them back, or whether it is in fact time for them to pass on for the afterlife and face judgement in death. 

Context for GMs: I designed this subclass in one of my campaigns where the BBEG was a Necromancer. The party had a Conquest Paladin and Life Cleric, both of whom I felt were becoming more enamoured with the idea of committing to the cause of fighting back the scourge of Undeath plaguing their lands. I offered them a choice of whether to consider a change in their sub-class to match this - the Cleric was offered this option (Which is largely a merger of Life and Grave domains individually) whilst I wrote an Oath of Crusade for the Paladin. In my campaign, the Cleric was already worshipping a homebrewed Life-Domain deity, with beliefs on the Undead in line with Kelemvor. They therefore have the responsibility to preserve life in the Forgotten Realms, whilst respecting the absolution of death - keeping both deities happy. 

Bonus Proficiency

When you choose this domain at 1st level, you gain proficiency with heavy armor.

Disciple of Life

Also starting at 1st level, your healing spells are more effective. Whenever you use a spell of 1st level or higher to restore hit points to a creature, the creature regains additional hit points equal to 2 + the spell’s level.

Eyes of the Grave

At 1st level, you gain the ability to occasionally sense the presence of the undead, whose existence is an insult to the natural cycle of life. As an action, you can open your awareness to magically detect undead. Until the end of your next turn, you know the location of any undead within 60 feet of you that isn't behind total cover and that isn't protected from divination magic. This sense doesn't tell you anything about a creature's capabilities or identity. 

You can use this feature a number of times equal to your Wisdom modifier (minimum of once). You regain all expended uses when you finish a long rest.

Channel Divinity: Preserve Life, Respect Death

Starting at 2nd level, you can use your Channel Divinity to heal the badly injured or to rebuke your foes.

As an action, you can use your Channel Divinity to either restore HP equal to five times your cleric level, or deal the same amount as either Necrotic or Radiant damage (your choice). Choose any creatures within 30 ft. of you, and divide the amount of damage or hit points among them. This feature can restore a creature to no more than half of its hit point maximum. You can’t use this feature to restore hit points to an undead or a construct.

Blessed Healer

Beginning at 6th level, the healing spells you cast on others heal you as well. When you cast a spell of 1st level or higher that restores hit points to a creature other than you, you regain hit points equal to 2 + the spell’s level.

Path to the Grave

Starting at 8th level, you can mark another creature’s life force for termination.

As an action, you choose one creature you can see within 30 feet of you, cursing it until the end of your next turn. The next time you or an ally of yours hits the cursed creature with an attack, the creature has vulnerability to all of that attack's damage, and then the curse ends.

Where a creature is immune to the next damage dealt to it after this ability is used, the damage is then calculated as if the creature was only resistant to that damage - not immune.  Where a creature is resistant to the next damage dealt to it after this ability is used, the damage is then calculated as if the creature was neither resistant nor vulnerable to that damage.

You can use this feature a number of times equal to half your Wisdom modifier rounded up (minimum of once). You regain all expended uses when you finish a long rest.

Supreme Healing

Starting at 17th level, when you would normally roll one or more dice to restore hit points with a spell, you instead use the highest number possible for each die. For example, instead of restoring 2d6 hit points to a creature, you restore 12.