Waste Not

Whenever an opponent discards a creature card, create a 2/2 black Zombie creature token.
Whenever an opponent discards a land card, add {B}{B}.
Whenever an opponent discards a noncreature, nonland card, draw a card.

Guul Draz Specter

Flying
Guul Draz Specter gets +3/+3 as long as an opponent has no cards in hand.
Whenever Guul Draz Specter deals combat damage to a player, that player discards a card.

Gonti, Lord of Luxury

Deathtouch
When Gonti, Lord of Luxury enters, look at the top four cards of target opponent's library, exile one of them face down, then put the rest on the bottom of that library in a random order. You may cast that card for as long as it remains exiled, and mana of any type can be spent to cast that spell.

Jin-Gitaxias, Core Augur

Flash
At the beginning of your end step, draw seven cards.
Each opponent's maximum hand size is reduced by seven.

Fell Specter

Flying
When Fell Specter enters, target opponent discards a card.
Whenever an opponent discards a card, that player loses 2 life.

Syphon Mind

Each other player discards a card. You draw a card for each card discarded this way.

Dimir Signet

Painful Quandary

Whenever an opponent casts a spell, that player loses 5 life unless they discard a card.

Hypnotic Specter

Flying
Whenever Hypnotic Specter deals damage to an opponent, that player discards a card at random.

Head Games

Target opponent puts the cards from their hand on top of their library. Search that player's library for that many cards. The player puts those cards into their hand, then shuffles.

Glasses of Urza

Zur's Weirding

Players play with their hands revealed.
If a player would draw a card, they reveal it instead. Then any other player may pay 2 life. If a player does, put that card into its owner's graveyard. Otherwise, that player draws a card.

Geth's Grimoire

Whenever an opponent discards a card, you may draw a card.

Telepathy

Your opponents play with their hands revealed.

Nebuchadnezzar

{X}, {T}: Choose a card name. Target opponent reveals X cards at random from their hand. Then that player discards all cards with that name revealed this way. Activate only during your turn.

Seer's Vision

Your opponents play with their hands revealed.
Sacrifice Seer's Vision: Look at target player's hand and choose a card from it. That player discards that card. Activate only as a sorcery.

Recoil

Return target permanent to its owner's hand. Then that player discards a card.

Liliana's Caress

Whenever an opponent discards a card, that player loses 2 life.

Bottomless Pit

At the beginning of each player's upkeep, that player discards a card at random.