Karlach, Fury of Avernus

Whenever you attack, if it's the first combat phase of the turn, untap all attacking creatures. They gain first strike until end of turn. After this phase, there is an additional combat phase.
Choose a Background (You can have a Background as a second commander.)

Pygmy Troll

Whenever Pygmy Troll becomes blocked by a creature, Pygmy Troll gets +1/+1 until end of turn.
{G}: Regenerate Pygmy Troll.

Cockatrice

Flying
Whenever Cockatrice blocks or becomes blocked by a non-Wall creature, destroy that creature at end of combat.

Mountain

Shinen of Life's Roar

All creatures able to block Shinen of Life's Roar do so.
Channel — {2}{G}{G}, Discard Shinen of Life's Roar: All creatures able to block target creature this turn do so.

You've Been Caught Stealing

Choose one —
• Threaten the Merchant — Each creature blocks this turn if able.
• Bribe the Guards — You create a Treasure token for each opponent who was dealt damage this turn. (It's an artifact with "{T}, Sacrifice this artifact: Add one mana of any color.")

Hungering Hydra

Hungering Hydra enters the battlefield with X +1/+1 counters on it.
Hungering Hydra can't be blocked by more than one creature.
Whenever Hungering Hydra is dealt damage, put that many +1/+1 counters on it. (It must survive the damage to get the counters.)

Taunting Elf

All creatures able to block Taunting Elf do so.

Corrosive Ooze

Whenever Corrosive Ooze blocks or becomes blocked by an equipped creature, destroy all Equipment attached to that creature at end of combat.

Retaliation

Creatures you control have "Whenever this creature becomes blocked by a creature, this creature gets +1/+1 until end of turn."

Lure

Enchant creature
All creatures able to block enchanted creature do so.

You Look Upon the Tarrasque

Choose one —
• Run and Hide — Prevent all combat damage that would be dealt to you and creatures you control this turn.
• Gather Your Courage — Target creature gets +5/+5 and gains indestructible until end of turn. All creatures your opponents control able to block that creature this turn do so.

Nessian Boar

All creatures able to block Nessian Boar do so.
Whenever Nessian Boar becomes blocked by a creature, that creature's controller draws a card.

Nacatl War-Pride

Nacatl War-Pride must be blocked by exactly one creature if able.
Whenever Nacatl War-Pride attacks, create X tokens that are copies of Nacatl War-Pride and that are tapped and attacking, where X is the number of creatures defending player controls. Exile the tokens at the beginning of the next end step.

Gorm the Great

Partner with Virtus the Veiled (When this creature enters the battlefield, target player may put Virtus into their hand from their library, then shuffle.)
Vigilance
Gorm the Great must be blocked if able, and Gorm must be blocked by two or more creatures if able.

Venom

Enchant creature
Whenever enchanted creature blocks or becomes blocked by a non-Wall creature, destroy the other creature at end of combat.

Defiler of Vigor

Trample
As an additional cost to cast green permanent spells, you may pay 2 life. Those spells cost {G} less to cast if you paid life this way. This effect reduces only the amount of green mana you pay.
Whenever you cast a green permanent spell, put a +1/+1 counter on each creature you control.

Tangle Asp

Whenever Tangle Asp blocks or becomes blocked by a creature, destroy that creature at end of combat.

Forest

Tempting Licid

{G}, {T}: Tempting Licid loses this ability and becomes an Aura enchantment with enchant creature. Attach it to target creature. You may pay {G} to end this effect.
All creatures able to block enchanted creature do so.

Questing Beast

Vigilance, deathtouch, haste
Questing Beast can't be blocked by creatures with power 2 or less.
Combat damage that would be dealt by creatures you control can't be prevented.
Whenever Questing Beast deals combat damage to an opponent, it deals that much damage to target planeswalker that player controls.

Revenge of the Hunted

Until end of turn, target creature gets +6/+6 and gains trample, and all creatures able to block it this turn do so.
Miracle {G} (You may cast this card for its miracle cost when you draw it if it's the first card you drew this turn.)

Horrible Hordes

Rampage 1 (Whenever this creature becomes blocked, it gets +1/+1 until end of turn for each creature blocking it beyond the first.)

Venomous Breath

Choose target creature. At this turn's next end of combat, destroy all creatures that blocked or were blocked by it this turn.

Noble Quarry

Bestow {5}{G} (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached to a creature.)
All creatures able to block Noble Quarry or enchanted creature do so.
Enchanted creature gets +1/+1.

Elven Warhounds

Whenever Elven Warhounds becomes blocked by a creature, put that creature on top of its owner's library.

Ravenous Slime

Ravenous Slime can't be blocked by creatures with power 2 or less.
If a creature an opponent controls would die, instead exile it and put a number of +1/+1 counters equal to that creature's power on Ravenous Slime.

Roar of Challenge

All creatures able to block target creature this turn do so.
Ferocious — That creature gains indestructible until end of turn if you control a creature with power 4 or greater.

Sylvan Basilisk

Whenever Sylvan Basilisk becomes blocked by a creature, destroy that creature.

Neyith of the Dire Hunt

Whenever one or more creatures you control fight or become blocked, draw a card.
At the beginning of combat on your turn, you may pay {2}{R/G}. If you do, double target creature's power until end of turn. That creature must be blocked this combat if able. ({R/G} can be paid with either {R} or {G}.)

Gorilla Berserkers

Trample; rampage 2 (Whenever this creature becomes blocked, it gets +2/+2 until end of turn for each creature blocking it beyond the first.)
Gorilla Berserkers can't be blocked except by three or more creatures.

Craw Giant

Trample
Rampage 2 (Whenever this creature becomes blocked, it gets +2/+2 until end of turn for each creature blocking it beyond the first.)

Venomous Dragonfly

Flying
Whenever Venomous Dragonfly blocks or becomes blocked by a creature, destroy that creature at end of combat.

General Marhault Elsdragon

Whenever a creature you control becomes blocked, it gets +3/+3 until end of turn for each creature blocking it.

Engulfing Slagwurm

Whenever Engulfing Slagwurm blocks or becomes blocked by a creature, destroy that creature. You gain life equal to that creature's toughness.

Snag

You may discard a Forest card rather than pay this spell's mana cost.
Prevent all combat damage that would be dealt by unblocked creatures this turn.