Agatha's Soul Cauldron

Take the graveyard-exile toolbox that lets creatures inherit dead things' abilities, and the obvious ancestor is Necrotic Ooze, which grafted the activated abilities of every creature card in your graveyard onto one body. This restructures that idea across three separate axes. The first ability turns your creatures into color-fixing engines for their own activated abilities, which is quiet infrastructure until you realize how many creature abilities cost off-color mana you would otherwise struggle to produce. The second is the real machine: rather than reading from your graveyard, it exiles creature cards into the Cauldron itself and lends their activated abilities to any creature you control carrying a +1/+1 counter, which means the ability library is stable, cannot be Bojuka Bogged away, and grows on your schedule. The third ability is the pump that feeds the second: exiling a creature card from any graveyard both loads the library and hands out the counter that unlocks it, and it doubles as generic graveyard hate at instant speed. What makes the design cohere is that each clause reinforces the others: the counter matters only because of the ability-sharing clause, the ability-sharing clause matters only because you have exiled something worth copying, and the mana-fixing matters only once you are activating creature abilities. That interlock is why it reads as a combo piece rather than a value engine, and why its exploits tend to be specific and violent rather than incremental.