Dauntless Dismantler

Artifacts your opponents control enter tapped.
{X}{X}{W}, Sacrifice Dauntless Dismantler: Destroy each artifact with mana value X.

Queen Kayla bin-Kroog

{4}, {T}: Discard all the cards in your hand, then draw that many cards. You may choose an artifact or creature card with mana value 1 you discarded this way, then do the same for artifact or creature cards with mana values 2 and 3. Return those cards to the battlefield. Activate only as a sorcery.

Archon of Emeria

Flying
Each player can't cast more than one spell each turn.
Nonbasic lands your opponents control enter tapped.

Archivist of Oghma

Flash
Whenever an opponent searches their library, you gain 1 life and draw a card.

Boromir, Warden of the Tower

Vigilance
Whenever an opponent casts a spell, if no mana was spent to cast it, counter that spell.
Sacrifice Boromir, Warden of the Tower: Creatures you control gain indestructible until end of turn. The Ring tempts you.

Aven Mindcensor

Flash
Flying
If an opponent would search a library, that player searches the top four cards of that library instead.