I'd bet that they are Valkyrie. They are even watching over a battle.
Also, the jagged lines in the background going up and down remind me of a certain world tree. A giant tree seems like it would play in pretty well with the idea of "branching" pathways that we will see on the remaining MDFCs
Shuk-Shuk the OldestXR
Legendary Creature - Elder Goblin Shaman
Shuk-Shuk enters the battlefield with X +1/+1 counters on it.
Whenever another Goblin you control dies, put a +1/+1 counter on Shuk-Shuk.
When Shuk-Shuk dies, you may distribute its +1/+1 counters among any number of Goblin creatures you control.
0/0
Forever LostUW
Enchantment - Aura
Enchant Creature
Whenever enchanted creature attacks, untap it and prevent all combat damage that it would deal this turn.
Experience Eternity4UUU
Sorcery
Look at all of the cards in your deck and put them back in any order.
Hymn of Fallen Ancients 3WW
Enchantment
Creatures you control get +X/+X, where X is the number of creature cards in your graveyard.
Shimmerscale the Awakened2RW
Legendary Creature - Dinosaur Knight RW: The next 1 damage that would be dealt to ~ is dealt to target creature or player instead.
3/5
Kor Sanctifiers3W
Creature - Kor Cleric
Kicker W
When ~ enters the battlefield, destroy target artifact or enchantment. Then, if it was kicked, return target artifact or enchantment from a graveyard to the battlefield.
2/4
Juma the Exhumer4GB
Legendary Creature - Praetor
Wheneve a creature you control dies, you may put another target creature from your graveyard onto the battlefield.
5/5
Also, the jagged lines in the background going up and down remind me of a certain world tree. A giant tree seems like it would play in pretty well with the idea of "branching" pathways that we will see on the remaining MDFCs
Legendary Creature - Elder Goblin Shaman
Shuk-Shuk enters the battlefield with X +1/+1 counters on it.
Whenever another Goblin you control dies, put a +1/+1 counter on Shuk-Shuk.
When Shuk-Shuk dies, you may distribute its +1/+1 counters among any number of Goblin creatures you control.
0/0
Creature - Fungus Giant
Trample
At the beginning
Enchantment - Aura
Enchant Creature
Whenever enchanted creature attacks, untap it and prevent all combat damage that it would deal this turn.
Experience Eternity 4UUU
Sorcery
Look at all of the cards in your deck and put them back in any order.
Hymn of Fallen Ancients 3WW
Enchantment
Creatures you control get +X/+X, where X is the number of creature cards in your graveyard.
Next:
Ground and Pound
Shelldock Ferryman
Legendary Creature - Dinosaur Knight
RW: The next 1 damage that would be dealt to ~ is dealt to target creature or player instead.
3/5
Next:
Dragon Archer
Barbarian Wizard
Creature - Siren Pirate
Flying
When ~ attacks, target creature can't block this turn.
2/1
Creature - Kor Cleric
Kicker W
When ~ enters the battlefield, destroy target artifact or enchantment. Then, if it was kicked, return target artifact or enchantment from a graveyard to the battlefield.
2/4
Next: Razor Golem
Instant
~ deals 3 damage to target attacking creature.
Next: The Hive
Creature - Elf Soldier
Sacrifice ~: Counter target black spell.
2/2
Next: Harbinger of Curses
Creature - Elemental
Flying
UW: Counter target ability that targets a creature you control.
4/4
Legendary Creature - Praetor
Wheneve a creature you control dies, you may put another target creature from your graveyard onto the battlefield.
5/5
Land
~ enters the battlefield tapped.
t: Add U to your mana pool.
UU,t: Add one mana of any color to your mana pool.
Artifact Creature - Myr
Whenever another Myr enters the battlefield, put a -1/-1 counter on it.
3/3
Mounds of the Unremembered
Land
1, T: Add BW to your mana pool.
BW, Sacrifice a Spirit: Gain life equal to the sacrificed creature's toughness.
Next: something with cmc 10 or more.
Creature - Imp
Flying
Whenever a Swamp enters the battlefield, ~ deals 1 damage to that land's controller.
3/3