Corpses of the Lost

Skeletons you control get +1/+0 and have haste.
When Corpses of the Lost enters the battlefield, create a 2/2 black Skeleton Pirate creature token.
At the beginning of your end step, if you descended this turn, you may pay 1 life. If you do, return Corpses of the Lost to its owner's hand. (You descended if a permanent card was put into your graveyard from anywhere.)

Crippling Fear

Choose a creature type. Creatures that aren't of the chosen type get -3/-3 until end of turn.

Gutterbones

Gutterbones enters the battlefield tapped.
{1}{B}: Return Gutterbones from your graveyard to your hand. Activate only during your turn and only if an opponent lost life this turn.

Underworld Sentinel

Whenever Underworld Sentinel attacks, exile target creature card from your graveyard.
When Underworld Sentinel dies, put all cards exiled with it onto the battlefield.

Immortal Servitude

Return each creature card with mana value X from your graveyard to the battlefield.

Duty-Bound Dead

Exalted (Whenever a creature you control attacks alone, that creature gets +1/+1 until end of turn.)
{3}{B}: Regenerate Duty-Bound Dead. (The next time this creature would be destroyed this turn, it isn't. Instead tap it, remove all damage from it, and remove it from combat.)

Out of the Tombs

At the beginning of your upkeep, put two eon counters on Out of the Tombs, then mill cards equal to the number of eon counters on it.
If you would draw a card while your library has no cards in it, instead return a creature card from your graveyard to the battlefield. If you can't, you lose the game.

Skeleton Crew

Each other creature you control that's a Skeleton or Pirate gets +1/+1.
Whenever one or more creature cards leave your graveyard, create a 2/2 black Skeleton Pirate creature token. (This ability triggers only from the battlefield.)
{5}{B}: Return Skeleton Crew from your graveyard to the battlefield tapped.

Spawning Pool

Spawning Pool enters the battlefield tapped.
{T}: Add {B}.
{1}{B}: Spawning Pool becomes a 1/1 black Skeleton creature with "{B}: Regenerate this creature" until end of turn. It's still a land. (If it regenerates, the next time it would be destroyed this turn, it isn't. Instead tap it, remove all damage from it, and remove it from combat.)

Champion of Stray Souls

{3}{B}{B}, {T}, Sacrifice X other creatures: Return X target creature cards from your graveyard to the battlefield.
{5}{B}{B}: Put Champion of Stray Souls on top of your library from your graveyard.

Golgari Guildgate

Golgari Guildgate enters the battlefield tapped.
{T}: Add {B} or {G}.

Pile On

Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.)
Destroy target creature or planeswalker. Surveil 2. (Look at the top two cards of your library, then put any number of them into your graveyard and the rest on top of your library in any order.)

Skeletal Swarming

Each Skeleton you control has trample, attacks each combat if able, and gets +X/+0, where X is the number of other Skeletons you control.
At the beginning of your end step, create a tapped 1/1 black Skeleton creature token. If a creature died this turn, create two of those tokens instead.

Swamp

Death-Priest of Myrkul

Skeletons, Vampires, and Zombies you control get +1/+1.
At the beginning of your end step, if a creature died this turn, you may pay {1}. If you do, create a 1/1 black Skeleton creature token.

Bone Dragon

Flying
{3}{B}{B}, Exile seven other cards from your graveyard: Return Bone Dragon from your graveyard to the battlefield tapped.