Abyssal Nocturnus

Whenever an opponent discards a card, Abyssal Nocturnus gets +2/+2 and gains fear until end of turn. (It can't be blocked except by artifact creatures and/or black creatures.)

Drainpipe Vermin

When Drainpipe Vermin dies, you may pay {B}. If you do, target player discards a card.

Cry of Contrition

Target player discards a card.
Haunt (When this spell card is put into a graveyard after resolving, exile it haunting target creature.)
When the creature Cry of Contrition haunts dies, target player discards a card.

Abyssal Specter

Flying
Whenever Abyssal Specter deals damage to a player, that player discards a card.

Swamp

Liliana's Caress

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

Horrifying Revelation

Target player discards a card, then mills a card.

Megrim

Whenever an opponent discards a card, Megrim deals 2 damage to that player.

Dark Ritual

Thoughtseize

Target player reveals their hand. You choose a nonland card from it. That player discards that card. You lose 2 life.

Hymn to Tourach

Target player discards two cards at random.

Painful Quandary

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