Dream Halls

Rather than pay the mana cost for a spell, its controller may discard a card that shares a color with that spell.

Aboshan, Cephalid Emperor

Tap an untapped Octopus you control: Tap target permanent.
{U}{U}{U}: Tap all creatures without flying.

Narset, Enlightened Master

First strike, hexproof
Whenever Narset, Enlightened Master attacks, exile the top four cards of your library. Until end of turn, you may cast noncreature spells from among those cards without paying their mana costs.

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.

Mirrorweave

Each other creature becomes a copy of target nonlegendary creature until end of turn.

Confusion in the Ranks

Whenever an artifact, creature, or enchantment enters, its controller chooses target permanent another player controls that shares a card type with it. Exchange control of those permanents.

Show and Tell

Each player may put an artifact, creature, enchantment, or land card from their hand onto the battlefield.

Repercussion

Whenever a creature is dealt damage, Repercussion deals that much damage to that creature's controller.

Worldpurge

Return all permanents to their owners' hands. Each player chooses up to seven cards in their hand, then shuffles the rest into their library. Each player loses all unspent mana.