Foundations Jumpstart

Creature — Human Shaman

Undergrowth — When this creature enters, target creature gains menace and gets +X/+0 until end of turn, where X is the number of creature cards in your graveyard. (It can't be blocked except by two or more creatures.)

Moodmark Painter

Instant

Undergrowth — Target creature gets -X/-X until end of turn, where X is the number of creature cards in your graveyard. If that creature would die this turn, exile it instead.

Necrotic Wound

Modern Horizons 3 Commander

Insect

Legendary Creature — Elf Shaman

Undergrowth — When Izoni enters, create a 1/1 black and green Insect creature token for each creature card in your graveyard.
{B}{G}, Sacrifice another creature: You gain 1 life and draw a card.

Izoni, Thousand-Eyed

Ravnica: Clue Edition

Creature — Insect Warrior

Reach
Undergrowth — When this creature enters, choose up to one target creature you don't control with flying. This creature gets +X/+0 until end of turn, where X is the number of creature cards in your graveyard, then you may have this creature fight that creature.

Kraul Harpooner

Commander Masters

Creature — Zombie Giant

Undergrowth — When this creature enters, it deals 1 damage to target opponent for each creature card in your graveyard.

Lotleth Giant

March of the Machine: The Aftermath

Sorcery

Undergrowth — Distribute X +1/+1 counters among any number of target creatures you control, where X is the number of creature cards in your graveyard as you cast this spell. Creatures you control gain vigilance until end of turn.

Undercity Upheaval

Jumpstart 2022

Creature — Insect Scout

Undergrowth — When this creature enters, you gain 1 life for each creature card in your graveyard.

Kraul Foragers

Guilds of Ravnica Promos

Undergrowth — Search your library for a black card with mana value less than or equal to the number of creature cards in your graveyard, reveal it, put it into your hand, then shuffle.

Mausoleum Secrets

Creature — Spider

Reach
Undergrowth — When you cast this spell, reveal the top X cards of your library, where X is the number of creature cards in your graveyard. You may put a green permanent card with mana value X or less from among them onto the battlefield. Put the rest on the bottom of your library in a random order.

Hatchery Spider

Guilds of Ravnica

Creature — Elf Warrior

Haste
Undergrowth — This creature enters with a +1/+1 counter on it for each creature card in your graveyard.

Golgari Raiders

Creature — Wurm

Undergrowth — When this creature enters, target creature gains vigilance and gets +X/+X until end of turn, where X is the number of creature cards in your graveyard.
This creature can't be blocked by more than one creature.

Vigorspore Wurm

Creature — Fungus Zombie

Undergrowth — This spell costs {1} less to cast for each creature card in your graveyard.
When this creature enters, return target land card from your graveyard to the battlefield.

Molderhulk

Undergrowth — This creature enters with a number of +1/+1 counters on it equal to the number of creature cards in your graveyard.