One or more stun counters on a permanent create a single replacement effect that stops the permanent from untapping
Amethyst Speaking of amethyst, the team at Ravensburger describes this as an otherworldly Ink of sorcerers, sages, and animated objects
That said, it has synergies here and there with loot effects or effects that require you to discard a card to play, so its a nice role-player in a discard-heavy strategy
With both of those on the battlefield, Restoration Angel and the Felidar Guardian can infinitely flicker each other and another creature you control to give you an endless amount of enters-the-battlefield triggers
These types of cards allow players to gain perfect information about their opponent's hand, don't cost a card in their hand to play (cause it gets replaced), and helps the player adjust their gameplay accordingly on that turn
Late game, when the cheaper spell isn't looking so hot you are far less constrained by mana and far more constrained by nominal power and cards