Avenge is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This spell costs less to cast if a player attacked you during their last turn. Destroy all creatures. You gain 1 life for each creature destroyed this way.

How it works : Avenge

Original English text — Wizards of the Coast / Scryfall

No additional ruling has been published for this card.

Interactions with other cards

These cards' text mentions Avenge.

Nighthawk Scavenger

Flying, deathtouch, lifelink Nighthawk Scavenger's power is equal to 1 plus the number of card types among cards in your opponents' graveyards.

Timin, Youthful Geist

Partner with Rhoda, Geist Avenger (When this creature enters, target player may put Rhoda into their hand from their library, then shuffle.) Flying At the beginning of each combat, tap up to one target creature.

Serra Avenger

You can't cast Serra Avenger during your first, second, or third turns of the game. Flying, vigilance

Young Deathclaws

Menace (This creature can't be blocked except by two or more creatures.) Each creature card in your graveyard has scavenge. The scavenge cost is equal to its mana cost. (Exile a creature card from your graveyard and pay its mana cost: Put a number of +1/+1 counters equal to that card's power on target creature. Scavenge only as a sorcery.)

Malanthrope

Flying Scavenge the Dead — When this creature enters, exile target player's graveyard. Put a +1/+1 counter on this creature for each creature card exiled this way.

Varolz, the Scar-Striped

Each creature card in your graveyard has scavenge. The scavenge cost is equal to its mana cost. (Exile a creature card from your graveyard and pay its mana cost: Put a number of +1/+1 counters equal to that card's power on target creature. Scavenge only as a sorcery.) Sacrifice another creature: Regenerate Varolz.

Avengers Disassembled

Choose one or both — • Avengers Disassembled deals 3 damage to each creature. • Destroy target land. Its controller may search their library for a basic land card, put it onto the battlefield tapped, then shuffle.

Boneyard Mycodrax

Boneyard Mycodrax's power and toughness are each equal to the number of other creature cards in your graveyard. Scavenge {4}{B} ({4}{B}, Exile this card from your graveyard: Put a number of +1/+1 counters equal to this card's power on target creature. Scavenge only as a sorcery.)

Dodgy Jalopy

Trample Dodgy Jalopy's power is equal to the greatest mana value among creatures you control. Crew 3 Scavenge {2}{G} ({2}{G}, Exile this card from your graveyard: Put a number of +1/+1 counters equal to this card's power on target creature. Scavenge only as a sorcery.)

Graf Rats

At the beginning of combat on your turn, if you both own and control this creature and a creature named Midnight Scavengers, exile them, then meld them into Chittering Host.

Hulkling, Young Avenger

Flying Whenever you cast a noncreature spell, Hulkling becomes a copy of up to one other target creature until end of turn, except his name is Hulkling, Young Avenger, he's 4/4, and he has flying and this ability.

Golgari Decoy

All creatures able to block this creature do so. Scavenge {3}{G}{G} ({3}{G}{G}, Exile this card from your graveyard: Put a number of +1/+1 counters equal to this card's power on target creature. Scavenge only as a sorcery.)