Shatter the Oath is excellent in Wilds of Eldraine no matter its mana cost, since its effect can hit practically any card the opponent might have
Ashcoat of the Shadow Swarm Ashcoat of the Shadow Swarm Azure Beastbinder Blightbelly Rat Bog Rats Burglar Rat Carrion Rats Chittering Rats Chittering Skitterling Crypt Rats Daggerfang Duo Deathmask Nezumi Dirty Rat Dirty Wererat Disease Carriers Diseased Vermin Drainpipe Vermin Earsplitting Rats Elephant-Rat Engine Rat Freya Crescent Gnat Miser Gnawing Vermin Gobhobbler Rats Graf Rats Greasefang, Okiba Boss Gutter Skulk Hellhole Rats Hideous Fleshwheeler Ichor Rats Infected Vermin Ink-Eyes, Servant of Oni Innocuous Rat Insatiable Frugivore Karumonix, the Rat King Kuro's Taken Lightshell Duo Locust Miser Lord Skitter's Butcher Lord Skitter, Sewer King Marrow-Gnawer Mind Drill Assailant Mindwhisker Muck Rats Mukotai Ambusher Nashi, Moon Sage's Scion Nashi, Moon's Legacy Nashi, Searcher in the Dark Nezumi Bladeblesser Nezumi Bone-Reader Nezumi Cutthroat Nezumi Freewheeler Nezumi Graverobber Nezumi Informant Nezumi Linkbreaker Nezumi Prowler Nezumi Road Captain Nezumi Ronin Nezumi Shadow-Watcher Nezumi Shortfang Nightwhorl Hermit Okiba-Gang Shinobi Old Flitterfang Ossuary Rats Pack Rat Persistent Marshstalker Pestilence Rats Plague Rats Rabid Rats Rancid Rats Rat Colony Rat King, Pale Piper
More over it is a total beating against you, having it is nice, not facing it is a mild perk
That means it works more like an instruction in the middle of a spell or ability, similar to draw a card
I could see it being a piece of Legendary Equipment
