Hullbreaker Horror

Flash
This spell can't be countered.
Whenever you cast a spell, choose up to one —
• Return target spell you don't control to its owner's hand.
• Return target nonland permanent to its owner's hand.

Cyclonic Rift

Return target nonland permanent you don't control to its owner's hand.
Overload {6}{U} (You may cast this spell for its overload cost. If you do, change "target" in its text to "each.")

Blatant Thievery

For each opponent, gain control of target permanent that player controls.

Don Andres, the Renegade

Each creature you control but don't own gets +2/+2, has menace and deathtouch, and is a Pirate in addition to its other types.
Whenever you cast a noncreature spell you don't own, create two tapped Treasure tokens.

Gonti, Night Minister

Whenever a player casts a spell they don't own, that player creates a Treasure token.
Whenever a creature deals combat damage to one of your opponents, its controller looks at the top card of that opponent's library and exiles it face down. They may play that card for as long as it remains exiled. Mana of any type can be spent to cast a spell this way.

Blue Sun's Twilight

Gain control of target creature with mana value X or less. If X is 5 or more, create a token that's a copy of that creature.

Thalakos Deceiver

Shadow (This creature can block or be blocked by only creatures with shadow.)
Whenever this creature attacks and isn't blocked, you may sacrifice it. If you do, gain control of target creature. (This effect lasts indefinitely.)

Nathan Drake, Treasure Hunter

First strike
You may spend mana as though it were mana of any color to cast spells you don't own or to activate abilities of permanents you control but don't own.
Whenever Nathan Drake attacks, exile the top card of each player's library. You may cast a spell from among those cards.

Olivia Voldaren

Flying
{1}{R}: Olivia Voldaren deals 1 damage to another target creature. That creature becomes a Vampire in addition to its other types. Put a +1/+1 counter on Olivia Voldaren.
{3}{B}{B}: Gain control of target Vampire for as long as you control Olivia Voldaren.

Tempted by the Oriq

For each opponent, gain control of up to one target creature or planeswalker that player controls with mana value 3 or less.

Archmage's Charm

Choose one —
• Counter target spell.
• Target player draws two cards.
• Gain control of target nonland permanent with mana value 1 or less.

Arvinox, the Mind Flail

Arvinox isn't a creature unless you control three or more permanents you don't own.
At the beginning of your end step, exile the bottom card of each opponent's library face down. For as long as those cards remain exiled, you may look at them, you may cast permanent spells from among them, and you may spend mana as though it were mana of any color to cast those spells.

Thieving Amalgam

At the beginning of each opponent's upkeep, you manifest the top card of that player's library. (Put it onto the battlefield face down as a 2/2 creature. Turn it face up any time for its mana cost if it's a creature card.)
Whenever a creature you control but don't own dies, its owner loses 2 life and you gain 2 life.

Garland, Royal Kidnapper

When Garland enters, target opponent becomes the monarch.
Whenever an opponent becomes the monarch, gain control of target creature that player controls for as long as they're the monarch.
Creatures you control but don't own get +2/+2 and can't be sacrificed.

Empress Galina

{U}{U}, {T}: Gain control of target legendary permanent. (This effect lasts indefinitely.)

Hellkite Tyrant

Flying, trample
Whenever this creature deals combat damage to a player, gain control of all artifacts that player controls.
At the beginning of your upkeep, if you control twenty or more artifacts, you win the game.

Dominate

Gain control of target creature with mana value X or less.

Mass Manipulation

Gain control of X target creatures and/or planeswalkers.

Tevesh Szat, Doom of Fools

+2: Create two 0/1 black Thrull creature tokens.
+1: You may sacrifice another creature or planeswalker. If you do, draw two cards, then draw another card if the sacrificed permanent was a commander.
−10: Gain control of all commanders. Put all commanders from the command zone onto the battlefield under your control.
Tevesh Szat, Doom of Fools can be your commander.
Partner

Nicol Bolas, Planeswalker

+3: Destroy target noncreature permanent.
−2: Gain control of target creature.
−9: Nicol Bolas deals 7 damage to target player or planeswalker. That player or that planeswalker's controller discards seven cards, then sacrifices seven permanents of their choice.

Fumble

Return target creature to its owner's hand. Gain control of all Auras and Equipment that were attached to it, then attach them to another creature.

Treasure Nabber

Whenever an opponent taps an artifact for mana, gain control of that artifact until the end of your next turn.

Aura Thief

Flying
When this creature dies, you gain control of all enchantments. (You don't get to move Auras.)

Invert Polarity

Choose target spell, then flip a coin. If you win the flip, gain control of that spell and you may choose new targets for it. If you lose the flip, counter that spell.

Beguiler of Wills

{T}: Gain control of target creature with power less than or equal to the number of creatures you control.

Ashiok, Sculptor of Fears

+2: Draw a card. Each player mills two cards.
−5: Put target creature card from a graveyard onto the battlefield under your control.
−11: Gain control of all creatures target opponent controls.

Memnarch

{1}{U}{U}: Target permanent becomes an artifact in addition to its other types. (This effect lasts indefinitely.)
{3}{U}: Gain control of target artifact. (This effect lasts indefinitely.)

Subjugate the Hobbits

Gain control of each noncommander creature with mana value 3 or less.

Agent of Treachery

When this creature enters, gain control of target permanent.
At the beginning of your end step, if you control three or more permanents you don't own, draw three cards.

Expropriate

Council's dilemma — Starting with you, each player votes for time or money. For each time vote, take an extra turn after this one. For each money vote, choose a permanent owned by the voter and gain control of it. Exile Expropriate.

Roil Elemental

Flying
Landfall — Whenever a land you control enters, you may gain control of target creature for as long as you control this creature.

Herald of Leshrac

Flying
Cumulative upkeep—Gain control of a land you don't control. (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
This creature gets +1/+1 for each land you control but don't own.
When this creature leaves the battlefield, each player gains control of each land they own that you control.

Laughing Jasper Flint

Creatures you control but don't own are Mercenaries in addition to their other types.
At the beginning of your upkeep, exile the top X cards of target opponent's library, where X is the number of outlaws you control. Until end of turn, you may cast spells from among those cards, and mana of any type can be spent to cast those spells.