Valki, God of Lies // Tibalt, Cosmic Impostor

The design ambition here is a two-mana creature that hides a seven-mana planeswalker inside it, and the mechanism connecting the two halves rewards the study. The front is a legitimate disruption piece: a 2/1 that strips a creature from each opponent's hand and can borrow it back as a body. But the modal double-faced frame means the back is castable directly for its own cost, and Tibalt, Cosmic Impostor's real payload is his emblem, which you get the moment he enters and which sticks around even after the planeswalker itself is answered. Everything Tibalt exiles becomes castable, and you may spend mana of any color to pay for those cards, so the +2 that exiles the top card off each library doubles as a resource engine rather than pure disruption. The ability exiles rather than mills, which is functionally the whole point; the emblem only reaches cards in exile.

The problem players seized on is that a modal card is only as balanced as its cheapest legal cast, and casting the back face honestly was always the expensive path, not the interesting one. Feed the front-face creature into an effect that puts a card onto the stack for free, and the seven-mana bill disappears. That interaction with cascade proved so warping that Wizards rewrote the comprehensive rules governing cascade itself rather than touch the card: the errata to how "cast" resolves off cascade traces directly back to this back door the front never advertised.