Merfolk has been a deck in Modern since the beginning, but it never broke through as one of the best decks of the format. With Modern Horizons 2 and the addition of Svyelun of Sea and Sky and Tide Shaper, it finally got both a good one-drop, forever missing in this archetype, as well as an extremely good three-drop tribal card that both protects your team, is hard to kill and draws a card every turn.
But that still wasn’t enough to bring Merfolk to a competitive level in Modern! Dominaria United is trying to make the case for it with Vodalian Hexcatcher, a flash lord that’s also a great combat trick as well as a way to pump your creatures in response to a Fury or a Fire // Ice. It also just so happens to be the best spell possible against cascade decks, as well as any combo spell-based deck in the format, which is a huge chunk of them!