Necrosynthesis

Enchant creature
Enchanted creature has "Whenever another creature dies, put a +1/+1 counter on this creature."
When enchanted creature dies, look at the top X cards of your library, where X is its power. Put one of those cards into your hand and the rest on the bottom of your library in a random order.

Elenda, the Dusk Rose

Lifelink
Whenever another creature dies, put a +1/+1 counter on Elenda, the Dusk Rose.
When Elenda dies, create X 1/1 white Vampire creature tokens with lifelink, where X is Elenda's power.

Orzhov Signet

Ashnod's Altar

Teysa, Orzhov Scion

Sacrifice three white creatures: Exile target creature.
Whenever another black creature you control dies, create a 1/1 white Spirit creature token with flying.

Land Tax

At the beginning of your upkeep, if an opponent controls more lands than you, you may search your library for up to three basic land cards, reveal them, put them into your hand, then shuffle.

Skullclamp

Equipped creature gets +1/-1.
Whenever equipped creature dies, draw two cards.
Equip {1}

Teysa Karlov

If a creature dying causes a triggered ability of a permanent you control to trigger, that ability triggers an additional time.
Creature tokens you control have vigilance and lifelink.

Murder Investigation

Enchant creature you control
When enchanted creature dies, create X 1/1 white Soldier creature tokens, where X is its power.