Thirst for Thirds
Instant (U)
Draw three cards, then discard two cards unless you discard a card with mana value 3.
Four is excessive. Two is short. Three will do.

Grasping Tentacle

Creature -- Kraken Tentacle (M)
At the beginning of your upkeep, create a 1/1 blue Tentacle creature token. Then if you control eight or more Tentacles, transform Grasping Tentacle.
, Tap an untapped Tentacle you control: Tap target creature.
1/1
///
Eight-Armed Scourge
Creature -- Kraken (M)
When this creature transforms into Eight-Armed Scourge, sacrifice eight Tentacles. Gain control of any number of tapped creatures with combined mana value 8 or less.
8/8

Corrupt Precinct-
Enchantment-Aura
Enchant Land
Whenever enchanted land is tapped for mana, investigate and create a treasure token.
There are few businesses more profitable than the law.

Blood-Sensing Shark
Creature - Shark (C)
Press the Advantage - Blood-Sensing Shark has defender unless an opponent has 10 or less life.
5/5

Changing Winds

Sorcery (M)
Gain control of target permanent an opponent controls, then return each other nonland permanent they control to its owner's hand.
"If you want to survive the storm, take my hand."

Echo Through Time-
Sorcery
Take an extra turn after this one. Exile Echo Through Time with five time counters on it. It gains suspend.
Some heroes are so great that the world itself remembers their names.

Time Dilation
Enchantment {M}
At the beginning of your upkeep, if it's not an extra turn, take an extra turn after this one.

sort of a ripoff of razor's card directly above, but I couldn't think of anything else

Disregard
Sorcery (C)
Target player discards two cards, then draws a card.
Some ideas aren't worth the effort.

Sengir's Attendant
Creature - Vampire (U)
Flying, Deathtouch, Lifelink.
Even the smallest member of his court is clothed with the finest gold.
1/1

Decadent Return

Sorcery (R)
Return target creature card from your graveyard to the battlefield. For each mana from a Treasure that was spent to cast this spell, that creature enters the battlefield with an additional +1/+1 counter on it. If only mana from Treasures was spent to cast this spell, that creature gains haste until end of turn.
"What's the point of being buried with all of your worldly riches if you don't get to enjoy them anymore?" —Vizzarai, Lich Lord

wrong game?

Starving Sovereign-
Creature-Vampire Noble
Lifelink, menace
At the beginning of your end step, if you didn't gain life this turn, sacrifice Starving Sovereign.
Mortal souls can never truly understand the hunger of the dead.
3/2

Decadent Return.

Sorcery (R)
Return target creature card from your graveyard to the battlefield. For each mana from a Treasure that was spent to cast this spell, that creature enters the battlefield with an additional +1/+1 counter on it. If only mana from Treasures was spent to cast this spell, that creature gains haste until end of turn.
"What's the point of being buried with all of your worldly riches if you don't get to enjoy them anymore?" —Vizzarai, Lich Lord

^ reanimators with (even minor) upside usually cost 5

Ruin and Renewal
Sorcery
Return target creature card from a graveyard to the battlefield under your control. If was spent to cast this spell, create a 1/1 green Saproling creature token. If was spent to cast this spell, gain 3 life.
"The Abzan get a bad rap. We accept death not because we like losing loved ones, but because we know it brings new life" - Marrit of House Emesh

Mantle of the Grave-
Enchantment-Aura
Enchant creature
Whenever Mantle of the Grave enters the battlefield or enchanted creature attacks, create two 2/2 black Zombie creature tokens.
In time, all things find death.

Seven Gates
Enchantment
Seven Gates enters the battlefield with seven gate counters.
Remove a gates counter from Seven Gates: choose 1
Create a black legendary 2/2 demon creature token with flying, lifeline, and deathtouch named Balantrandir.
Draw 2 cards and lose 2 life.

Journey of the Damned
Enchantment {M}
When Journey of the Damned enters the battlefield, exile all creatures.
At the beginning of your upkeep, you may return a creature card exiled with Journey of the Damned to the battlefield under your control. If you do, it gains haste until end of turn.

Open The Final Portal-
Sorcery
Destroy all creatures. Each player discards their hand. Create a 9/9 legendary black Demon creature token named Ezereth, Keeper of the Barren Flame.
The being looked down upon the world, its eyes wreathed in blackened fire. It felt no pity. It would grant no mercy.

Unsheath the flame sword.
Instant
Target creature gets +1/+0 and first strike until end of turn.
Unsheath the flame sword deals 1 damage to each creature blocking or blocked by that creature.

Coalrunner-
Creature-Goblin Shaman
Haste, menace
Whenever Coalrunner attacks, target non-attacking creature you control becomes an attacking creature.
"Akivenraza shows us many paths, but none that are safe to walk."
1/1

Armored Battle Steed
Creature - Horse (M)
Whenever an Equipment enters the battlefield under your control, you may attach it to Armored Battle Steed. If you do, Armored Battle Steed deals 2 damage to target player.
3/3