Quietus Spike

Equipped creature has deathtouch.
Whenever equipped creature deals combat damage to a player, that player loses half their life, rounded up.
Equip {3}

Infectious Horror

Whenever Infectious Horror attacks, each opponent loses 2 life.

Warlock Class

(Gain the next level as a sorcery to add its ability.)
At the beginning of your end step, if a creature died this turn, each opponent loses 1 life.
{1}{B}: Level 2
When this Class becomes level 2, look at the top three cards of your library. Put one of them into your hand and the rest into your graveyard.
{6}{B}: Level 3
At the beginning of your end step, each opponent loses life equal to the life they lost this turn. (Damage causes loss of life.)

Blitzwing, Adaptive Assailant

Living metal (During your turn, this Vehicle is also a creature.)
At the beginning of combat on your turn, choose flying or indestructible at random. Blitzwing gains that ability until end of turn.
Whenever Blitzwing deals combat damage to a player, convert it.

Blitzwing, Cruel Tormentor

More Than Meets the Eye {3}{B} (You may cast this card converted for {3}{B}.)
At the beginning of your end step, target opponent loses life equal to the life that player lost this turn. If no life is lost this way, convert Blitzwing.

Blitzwing, Cruel Tormentor // Blitzwing, Adaptive Assailant

Dire Fleet Ravager

Menace, deathtouch
When Dire Fleet Ravager enters, each player loses a third of their life, rounded up.

Illuminor Szeras

Secrets of the Soul — {T}, Sacrifice another creature: Add an amount of {B} equal to the sacrificed creature's mana value.

Fraying Omnipotence

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