Optic Node1 Artifact
When Optic Node enters the battlefield, draw a card. 1, T: Mana in your mana pool becomes the color or colors of your choice within your commander's color identity.
Something like this would get more and more relevant as the game progressed, as EDH games tend to do.
The Archenemy7 Permanent
Spend at least 5 colored mana on the Archenemy.
When the Archenemy enters the battlefield, each player exiles 2 cards from the top of his or her library and adds it to the Archenemy's hand face down.
Treat the Archenemy as a player with 15 life that takes a turn after you.
At the beginning of the Archenemy's turn, each player exiles a card from the top of his or her library and adds it to the Archenemy's hand face down.
During the Archenemy's main phase, for each of its opponents, it plays a card from its hand at random without paying that card's mana cost for which all player targets of all activated abilities will be that opponent, and for which all permanent targets will be targets of that opponent's choice.
During the Archenemy's declare attackers step, for each creature it controls, flip a coin. If heads, that creature attacks a random opponent this turn.
Archangel of Purity4WW Creature - Angel
Flying
When Archangel of Purity blocks or is blocked by a creature, if that creature is blue, black, red or green, destroy that creature. 5/5
Next: Sunflash Mace
Mogg Festival
Red Wine // White Wine
Swordsman of SevensU Creature - Human Warrior
When Swordsman of Sevens enters the battlefield, if an opponent has exactly 7 life, you may return target creature that player controls to its owner's hand. He wields a blade of aether as unpredictable as it is transparent. 1/1
Theurgy X(Sacrifice this creature: Add X to your mana pool. Use this mana only to cast creature spells. Activate this ability only during your main phase.)
Pious Ranger1WG Creature - Elf Knight
First strike, flanking
Theurgy WG(Sacrifice Pious Ranger: Add WG to your mana pool. Use this mana only to cast creature spells. Activate this ability only during your main phase.) 2/2
Touched Thrull1WB Creature - Thrull
Theurgy W(Sacrifice Touched Thrull: Add W to your mana pool. Use this mana only to cast creature spells. Activate this ability only during your main phase.)
If the mana produced by Touched Thrull's Theurgy ability paid for the cost of a spell with converted mana cost 4 or greater, return Touched Thrull to the battlefield at the beginning of the next end step. 1/1
Vedalken Godseekers1WU Creature - Vedalken Wizard
Theurgy WU(Sacrifice Vedalken Godseekers: Add WU to your mana pool. Use this mana only to cast creature spells. Activate this ability only during your main phase.)
When Vedalken Godseekers dies, you may discard a card. If you do, look at the top X cards of your library, where X is the amount of mana in your mana pool. You may reveal a creature card with converted mana cost X or less. If you do, put it in your hand, then put the rest of the cards you looked at in this way at the bottom of your library. 1/3
Fiery SupplicantWR Creature - Beast
Haste
Theurgy WRRR(Sacrifice Fiery Supplicant: Add WRRR to your mana pool. Use this mana only to cast creature spells. Activate this ability only during your main phase.)
Whenever you sacrifice Fiery Supplicant, if you activated its Theurgy ability, it deals 4 damage to you. 1/1
Hobgoblin's Pitchfork2 Artifact - Equipment
Equipped creature gets +1/+1 and has first strike.
Unattach Hobgoblin's Pitchfork: Hobgoblin's Pitchfork deals 1 damage to target Kithkin creature. Tap that creature.
Equip 2
Spiritual Dominance7UW Enchantment
When Spiritual Dominance enters the battlefield, each player sacrifices all non-Spirit creatures he or she controls, then returns all Spirit creature cards from the graveyard to the battlefield.
Spirits you control get +1/+0 and have hexproof and piracy. (Whenever a creature with piracy deals combat damage to a player, its controller may draw a card.)
SPQRWWRR Enchantment
White and red creatures you control have double strike, get +1/+0, and have "Sacrifice this creature: The next time a source of your choice would deal damage to this creature's controller this turn, prevent that damage."
Whenever you control no white and red creatures, sacrifice SPQR.
(The Romans were a real Borosy empire, don't you think? Or, more likely, the Boros are very Roman.)
Anti-Time Continuum1UBR Enchantment
When Anti-Time Continuum enters the battlefield, reverse the game's turn order.
When Anti-Time Continuum leaves the battlefield, reverse the game"s turn order.
Whenever a player would draw a card, that player discards a card instead.
Whenever no player has cards in their hand, sacrifice Anti-Time Continuum.
Next:
Decanisphere
Blade of the Guildless
Wayward Mercenaries
Instant
As an additional cost to Falling Rocks, sacrifice a Mountain.
Falling Rocks deals 3 damage to target creature or player.
Next:
Grotesque Transformation
Nostalgic Knights
King's Detachment
Black Moon's Zenith
Artifact
When Optic Node enters the battlefield, draw a card.
1, T: Mana in your mana pool becomes the color or colors of your choice within your commander's color identity.
Something like this would get more and more relevant as the game progressed, as EDH games tend to do.
Artifact - Equipment
Equipped creature has "T: Target player loses 1 life."
Equip 1
Next:
Unholy Matrimony
Dupree Scornblade, Blacksail Rider
Form Phalanx
Artifact
Hexproof
T: Exile target artifact you control.
,T: Return an artifact card exiled with Bag of Holding to the battlefield.
Next:
Red Wine // White Wine
Relentless Troop
Moonflash Blade
Permanent
Spend at least 5 colored mana on the Archenemy.
When the Archenemy enters the battlefield, each player exiles 2 cards from the top of his or her library and adds it to the Archenemy's hand face down.
Treat the Archenemy as a player with 15 life that takes a turn after you.
At the beginning of the Archenemy's turn, each player exiles a card from the top of his or her library and adds it to the Archenemy's hand face down.
During the Archenemy's main phase, for each of its opponents, it plays a card from its hand at random without paying that card's mana cost for which all player targets of all activated abilities will be that opponent, and for which all permanent targets will be targets of that opponent's choice.
During the Archenemy's declare attackers step, for each creature it controls, flip a coin. If heads, that creature attacks a random opponent this turn.
Next:
Otarian Pirates
Mutinous Grumblings
Secession
Sorcery
Broken Wing deals 1 damage to target creature with flying. Tap that creature. It loses flying until end of turn.
Next:
Red Wine // White Wine
Sunflash Mace
Sorcery
For each creature, sacrifice it unless its controller pays life equal to its converted mana cost.
Next: G
Creature - Angel
Flying
When Archangel of Purity blocks or is blocked by a creature, if that creature is blue, black, red or green, destroy that creature.
5/5
Next:
Sunflash Mace
Mogg Festival
Red Wine // White Wine
Creature - Human Warrior
When Swordsman of Sevens enters the battlefield, if an opponent has exactly 7 life, you may return target creature that player controls to its owner's hand.
He wields a blade of aether as unpredictable as it is transparent.
1/1
Next: R
Creature - Elf Knight
First strike, flanking
Theurgy WG (Sacrifice Pious Ranger: Add WG to your mana pool. Use this mana only to cast creature spells. Activate this ability only during your main phase.)
2/2
Creature - Thrull
Theurgy W (Sacrifice Touched Thrull: Add W to your mana pool. Use this mana only to cast creature spells. Activate this ability only during your main phase.)
If the mana produced by Touched Thrull's Theurgy ability paid for the cost of a spell with converted mana cost 4 or greater, return Touched Thrull to the battlefield at the beginning of the next end step.
1/1
Creature - Vedalken Wizard
Theurgy WU (Sacrifice Vedalken Godseekers: Add WU to your mana pool. Use this mana only to cast creature spells. Activate this ability only during your main phase.)
When Vedalken Godseekers dies, you may discard a card. If you do, look at the top X cards of your library, where X is the amount of mana in your mana pool. You may reveal a creature card with converted mana cost X or less. If you do, put it in your hand, then put the rest of the cards you looked at in this way at the bottom of your library.
1/3
Creature - Beast
Haste
Theurgy WRRR (Sacrifice Fiery Supplicant: Add WRRR to your mana pool. Use this mana only to cast creature spells. Activate this ability only during your main phase.)
Whenever you sacrifice Fiery Supplicant, if you activated its Theurgy ability, it deals 4 damage to you.
1/1
Next: Tedium, Exhaust, Worldly, Warmth
Sorcery
Destroy target nonblack creature.
Sorcery
Destroy target artifact.
Next: W
Artifact - Equipment
Equipped creature gets +1/+1 and has first strike.
Unattach Hobgoblin's Pitchfork: Hobgoblin's Pitchfork deals 1 damage to target Kithkin creature. Tap that creature.
Equip 2
Next:
Cone of Cold
Mana Shield
Familiar's Collar
Enchantment
When Spiritual Dominance enters the battlefield, each player sacrifices all non-Spirit creatures he or she controls, then returns all Spirit creature cards from the graveyard to the battlefield.
Spirits you control get +1/+0 and have hexproof and piracy. (Whenever a creature with piracy deals combat damage to a player, its controller may draw a card.)
Next: 3
Enchantment
White and red creatures you control have double strike, get +1/+0, and have "Sacrifice this creature: The next time a source of your choice would deal damage to this creature's controller this turn, prevent that damage."
Whenever you control no white and red creatures, sacrifice SPQR.
(The Romans were a real Borosy empire, don't you think? Or, more likely, the Boros are very Roman.)
Next:
Intelligent Illusion
Jester's Boots
Wayward Mercenaries
Enchantment
When Anti-Time Continuum enters the battlefield, reverse the game's turn order.
When Anti-Time Continuum leaves the battlefield, reverse the game"s turn order.
Whenever a player would draw a card, that player discards a card instead.
Whenever no player has cards in their hand, sacrifice Anti-Time Continuum.
Next:
Decanisphere
Blade of the Guildless
Wayward Mercenaries