Teferi's Imp

Flying
Phasing (This phases in or out before you untap during each of your untap steps. While it's phased out, it's treated as though it doesn't exist.)
Whenever Teferi's Imp phases out, discard a card.
Whenever Teferi's Imp phases in, draw a card.

Persecute

Choose a color. Target player reveals their hand and discards all cards of that color.

Kemuri-Onna

When Kemuri-Onna enters, target player discards a card.
Whenever you cast a Spirit or Arcane spell, you may return Kemuri-Onna to its owner's hand.

Blackmail

Target player reveals three cards from their hand and you choose one of them. That player discards that card.

Forget

Target player discards two cards, then draws as many cards as they discarded this way.

Firestorm

As an additional cost to cast this spell, discard X cards.
Firestorm deals X damage to each of X targets.

Coercion

Target opponent reveals their hand. You choose a card from it. That player discards that card.

Three Tragedies

Dream Halls

Rather than pay the mana cost for a spell, its controller may discard a card that shares a color with that spell.