Silent Arbiter

No more than one creature can attack each combat.
No more than one creature can block each combat.

Omen Machine

Players can't draw cards.
At the beginning of each player's draw step, that player exiles the top card of their library. If it's a land card, the player puts it onto the battlefield. Otherwise, the player casts it without paying its mana cost if able.

Fractured Identity

Exile target nonland permanent. Each player other than its controller creates a token that's a copy of it.

Perplexing Chimera

Whenever an opponent casts a spell, you may exchange control of this creature and that spell. If you do, you may choose new targets for the spell. (If the spell becomes a permanent, you control that permanent.)

Tyrant of Discord

When this creature enters, target opponent chooses a permanent they control at random and sacrifices it. If a nonland permanent is sacrificed this way, repeat this process.

Creepy Doll

Indestructible
Whenever this creature deals combat damage to a creature, flip a coin. If you win the flip, destroy that creature.

Knowledge Pool

Imprint — When this artifact enters, each player exiles the top three cards of their library.
Whenever a player casts a spell from their hand, that player exiles it. If the player does, they may cast a spell from among other cards exiled with this artifact without paying its mana cost.

Hive Mind

Whenever a player casts an instant or sorcery spell, each other player copies that spell. Each of those players may choose new targets for their copy.

Dovescape

({W/U} can be paid with either {W} or {U}.)
Whenever a player casts a noncreature spell, counter that spell. That player creates X 1/1 white and blue Bird creature tokens with flying, where X is the spell's mana value.