Tombstone Stairwell

Cumulative upkeep {1}{B} (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
At the beginning of each upkeep, if Tombstone Stairwell is on the battlefield, each player creates a 2/2 black Zombie creature token with haste named Tombspawn for each creature card in their graveyard.
At the beginning of each end step and when Tombstone Stairwell leaves the battlefield, destroy all tokens created with Tombstone Stairwell. They can't be regenerated.

Mindcrank

Whenever an opponent loses life, that player mills that many cards. (Damage causes loss of life.)

Demonic Tutor

Search your library for a card, put that card into your hand, then shuffle.

Opposition Agent

Flash
You control your opponents while they're searching their libraries.
While an opponent is searching their library, they exile each card they find. You may play those cards for as long as they remain exiled, and you may spend mana as though it were mana of any color to cast them.

Takenuma, Abandoned Mire

{T}: Add {B}.
Channel — {3}{B}, Discard Takenuma, Abandoned Mire: Mill three cards, then return a creature or planeswalker card from your graveyard to your hand. This ability costs {1} less to activate for each legendary creature you control.

Dark Deal

Each player discards all the cards in their hand, then draws that many cards minus one.

Chains of Mephistopheles

If a player would draw a card except the first one they draw in each of their draw steps, that player discards a card instead. If the player discards a card this way, they draw a card. If the player doesn't discard a card this way, they mill a card.

Vampiric Tutor

Search your library for a card, then shuffle and put that card on top. You lose 2 life.

Perpetual Timepiece

{T}: Mill two cards.
{2}, Exile Perpetual Timepiece: Shuffle any number of target cards from your graveyard into your library.

Altar of Dementia

Sacrifice a creature: Target player mills cards equal to the sacrificed creature's power.

Altar of the Brood

Whenever another permanent you control enters, each opponent mills a card.

Entomb

Search your library for a card, put that card into your graveyard, then shuffle.

Grim Tutor

Search your library for a card, put that card into your hand, then shuffle. You lose 3 life.

Each player loses a third of their life, then discards a third of the cards in their hand, then sacrifices a third of the creatures they control of their choice, then sacrifices a third of the lands they control of their choice. Round up each time.

Imperial Seal

Syr Konrad, the Grim

Whenever another creature dies, or a creature card is put into a graveyard from anywhere other than the battlefield, or a creature card leaves your graveyard, Syr Konrad, the Grim deals 1 damage to each opponent.
{1}{B}: Each player mills a card. (They each put the top card of their library into their graveyard.)