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.

Forest

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.)

Swamp

Reassembling Skeleton

{1}{B}: Return Reassembling Skeleton from your graveyard to the battlefield tapped.

Dread Osseosaur

Menace
Whenever Dread Osseosaur enters the battlefield or attacks, you may mill two cards. (You may put the top two cards of your library into your graveyard.)

Visage of Dread

When Visage of Dread enters the battlefield, target opponent reveals their hand. You choose an artifact or creature card from it. That player discards that card.
Craft with two creatures {5}{B} ({5}{B}, Exile this artifact, Exile the two from among creatures you control and/or creature cards in your graveyard: Return this card transformed under its owner's control. Craft only as a sorcery.)

Visage of Dread // Dread Osseosaur

Bitter Triumph

As an additional cost to cast this spell, discard a card or pay 3 life.
Destroy target creature or planeswalker.

Deathless Knight

Haste
When you gain life for the first time each turn, return Deathless Knight from your graveyard to your hand.

Vampiric Rites

{1}{B}, Sacrifice a creature: You gain 1 life and draw a card.

Assassin's Trophy

Destroy target permanent an opponent controls. Its controller may search their library for a basic land card, put it onto the battlefield, then shuffle.

Case of the Stashed Skeleton

When this Case enters the battlefield, create a 2/1 black Skeleton creature token and suspect it. (It has menace and can't block.)
To solve — You control no suspected Skeletons. (If unsolved, solve at the beginning of your end step.)
Solved — {1}{B}, Sacrifice this Case: Search your library for a card, put it into your hand, then shuffle. Activate only as a sorcery.

Eaten Alive

As an additional cost to cast this spell, sacrifice a creature or pay {3}{B}.
Exile target creature or planeswalker.

Corrupted Conviction

As an additional cost to cast this spell, sacrifice a creature.
Draw two cards.

Death Baron

Skeletons you control and other Zombies you control get +1/+1 and have deathtouch.

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.