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.)

Agent of Stromgald

Ember Gale

Creatures target player controls can't block this turn. Ember Gale deals 1 damage to each white and/or blue creature that player controls.

Torrent of Souls

Return up to one target creature card from your graveyard to the battlefield if {B} was spent to cast this spell. Creatures target player controls get +2/+0 and gain haste until end of turn if {R} was spent to cast this spell. (Do both if {B}{R} was spent.)

Reroute

Change the target of target activated ability with a single target. (Mana abilities can't be targeted.)
Draw a card.

Stromgald Cabal

{T}, Pay 1 life: Counter target white spell.

War Cadence

{X}{R}: This turn, creatures can't block unless their controller pays {X} for each blocking creature they control.

Blood Lust

If target creature has toughness 5 or greater, it gets +4/-4 until end of turn. Otherwise, it gets +4/-X until end of turn, where X is its toughness minus 1.

Dauthi Trapper

{T}: Target creature gains shadow until end of turn. (It can block or be blocked by only creatures with shadow.)

Gorilla War Cry

Cast this spell only during combat before blockers are declared.
All creatures gain menace until end of turn. (They can't be blocked except by two or more creatures.)
Draw a card at the beginning of the next turn's upkeep.

Recover

Return target creature card from your graveyard to your hand.
Draw a card.

Oubliette

When Oubliette enters the battlefield, target creature phases out until Oubliette leaves the battlefield. Tap that creature as it phases in this way. (Auras and Equipment phase out with it. While permanents are phased out, they're treated as though they don't exist.)

Black Market

Whenever a creature dies, put a charge counter on Black Market.
At the beginning of your precombat main phase, add {B} for each charge counter on Black Market.

Blessing of Leeches

Flash
Enchant creature
At the beginning of your upkeep, you lose 1 life.
{0}: Regenerate enchanted creature.

War Barge

{3}: Target creature gains islandwalk until end of turn. When War Barge leaves the battlefield this turn, destroy that creature. A creature destroyed this way can't be regenerated. (A creature with islandwalk can't be blocked as long as defending player controls an Island.)

Shatter

Mystic Compass

{1}, {T}: Target land becomes the basic land type of your choice until end of turn.

Scarab of the Unseen

{T}, Sacrifice Scarab of the Unseen: Return all Auras attached to target permanent you own to their owners' hands. Draw a card at the beginning of the next turn's upkeep.

Mountain

Thrull Wizard

{1}{B}: Counter target black spell unless that spell's controller pays {B} or {3}.

Goblin War Drums

Creatures you control have menace. (They can't be blocked except by two or more creatures.)

Evil Presence

Word of Binding

Eternal Warrior

Enchant creature
Enchanted creature has vigilance.

Deathgrip

Unnatural Speed

Target creature gains haste until end of turn.

Thrull Retainer

Enchant creature
Enchanted creature gets +1/+1.
Sacrifice Thrull Retainer: Regenerate enchanted creature.

Sisters of the Flame

Terror

Destroy target nonartifact, nonblack creature. It can't be regenerated.

Fire Diamond

Fire Diamond enters the battlefield tapped.
{T}: Add {R}.

The Brute

Enchant creature
Enchanted creature gets +1/+0.
{R}{R}{R}: Regenerate enchanted creature.

Burrowing

Enchant creature
Enchanted creature has mountainwalk. (It can't be blocked as long as defending player controls a Mountain.)

Guild Artisan

Commander creatures you own have "Whenever this creature attacks a player, if no opponent has more life than that player, you create two Treasure tokens." (They're artifacts with "{T}, Sacrifice this artifact: Add one mana of any color.")

Power Matrix

{T}: Target creature gets +1/+1 and gains flying, first strike, and trample until end of turn.

Fear

Enchant creature (Target a creature as you cast this. This card enters the battlefield attached to that creature.)
Enchanted creature has fear. (It can't be blocked except by artifact creatures and/or black creatures.)

Chariot of the Sun

{2}, {T}: Until end of turn, target creature you control gains flying and has base toughness 1.

Gravedigger

When Gravedigger enters the battlefield, you may return target creature card from your graveyard to your hand.

Dark Ritual

Goblin War Paint

Enchant creature
Enchanted creature gets +2/+2 and has haste.

Red Elemental Blast

Choose one —
• Counter target blue spell.
• Destroy target blue permanent.

Bestial Fury

Enchant creature
When Bestial Fury enters the battlefield, draw a card at the beginning of the next turn's upkeep.
Whenever enchanted creature becomes blocked, it gets +4/+0 and gains trample until end of turn.

Builder's Bane

Destroy X target artifacts. Builder's Bane deals damage to each player equal to the number of artifacts they controlled that were put into a graveyard this way.

Raise Dead

Return target creature card from your graveyard to your hand.

Cinder Cloud

Destroy target creature. If a white creature dies this way, Cinder Cloud deals damage to that creature's controller equal to the creature's power.

Crimson Mage

{R}: Target creature you control gains haste until end of turn. (It can attack and {T} this turn.)

Mana Cache

At the beginning of each player's end step, put a charge counter on Mana Cache for each untapped land that player controls.
Remove a charge counter from Mana Cache: Add {C}. Any player may activate this ability but only during their turn before the end step.

Orcish Farmer

{T}: Target land becomes a Swamp until its controller's next untap step.

Agent of the Shadow Thieves

Commander creatures you own have "Whenever this creature attacks a player, if no opponent has more life than that player, put a +1/+1 counter on this creature. It gains deathtouch and indestructible until end of turn."

Imposing Visage

Enchant creature
Enchanted creature has menace. (It can't be blocked except by two or more creatures.)

Unearthly Blizzard

Up to three target creatures can't block this turn.

Aftershock

Destroy target artifact, creature, or land. Aftershock deals 3 damage to you.

Swamp

Leshrac's Rite

Enchant creature
Enchanted creature has swampwalk. (It can't be blocked as long as defending player controls a Swamp.)

Crystal Chimes

{3}, {T}, Sacrifice Crystal Chimes: Return all enchantment cards from your graveyard to your hand.

Erratic Portal

{1}, {T}: Return target creature to its owner's hand unless its controller pays {1}.

Detonate

Destroy target artifact with mana value X. It can't be regenerated. Detonate deals X damage to that artifact's controller.

Rakdos Cluestone

{T}: Add {B} or {R}.
{B}{R}, {T}, Sacrifice Rakdos Cluestone: Draw a card.

Filth

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
As long as Filth is in your graveyard and you control a Swamp, creatures you control have swampwalk.

Mages' Contest

You and target spell's controller bid life. You start the bidding with a bid of 1. In turn order, each player may top the high bid. The bidding ends if the high bid stands. The high bidder loses life equal to the high bid. If you win the bidding, counter that spell.

Mob Mentality

Enchant creature
Enchanted creature has trample.
Whenever all non-Wall creatures you control attack, enchanted creature gets +X/+0 until end of turn, where X is the number of attacking creatures.

Sisay's Ring

Panic

Cast this spell only during combat before blockers are declared.
Target creature can't block this turn.
Draw a card at the beginning of the next turn's upkeep.

Cave People

Whenever Cave People attacks, it gets +1/-2 until end of turn.
{1}{R}{R}, {T}: Target creature gains mountainwalk until end of turn. (It can't be blocked as long as defending player controls a Mountain.)