39 cards where the text includes “whenever an opponent casts a spell”

Ammit Eternal (Hour of Devastation #57)
Bloodvial Purveyor (Innistrad: Crimson Vow #98)
Boromir, Warden of the Tower (The Lord of the Rings: Tales of Middle-earth #4)
Captured by the Consulate (Kaladesh #8)
Chancellor of the Annex (New Phyrexia #6)
Counterbalance (Coldsnap #31)
Decree of Silence (Scourge #32)
Deep-Sea Kraken (Duel Decks: Mind vs. Might #3)
Dragonlair Spider (Planechase Anthology #87)
Forced Fruition (Lorwyn #66)
Ghostly Pilferer (Outlaws of Thunder Junction Commander #98)
Hermit of the Natterknolls // Lone Wolf of the Natterknolls (Shadows over Innistrad #209)
Hermit of the Natterknolls // Lone Wolf of the Natterknolls (Shadows over Innistrad #209)
Hunting Grounds (Dominaria Remastered #191)
Ishai, Ojutai Dragonspeaker (Commander Anthology Volume II #12)
Kaervek the Merciless (Double Masters 2022 #236)
Lavinia, Azorius Renegade (Ravnica Remastered #195)
Lurking Predators (Jumpstart #410)
Memory Erosion (Commander Anthology Volume II #45)
Modular Monstrosity (Unstable #155)
Mogg Sentry (Ninth Edition #204)
Mondronen Shaman // Tovolar's Magehunter (Dark Ascension #98)
Mondronen Shaman // Tovolar's Magehunter (Dark Ascension #98)
Painful Quandary (The Brothers' War #111)
Pardic Dragon (Modern Masters #124)
Perplexing Chimera (Born of the Gods #48)
Powerbalance (Modern Horizons 3 #131)
Reparations (Mirage #278)
Rhystic Study (Jumpstart #169)
Sauron, the Dark Lord (The Lord of the Rings: Tales of Middle-earth #224)
Scalding Viper // Steam Clean (Wilds of Eldraine #235)
Silverquill Silencer (Strixhaven: School of Mages #234)
Spelltithe Enforcer (Guildpact #18)
Sunscorch Regent (March of the Machine Commander #209)
Swindler's Scheme (New Capenna Commander #88)
Talion, the Kindly Lord (Wilds of Eldraine #215)
Taurean Mauler (Neon Dynasty Commander #111)
Thundering Mightmare (Crimson Vow Commander #37)
Unstable Glyphbridge // Sandswirl Wanderglyph (The Lost Caverns of Ixalan #41)
Unstable Glyphbridge // Sandswirl Wanderglyph (The Lost Caverns of Ixalan #41)
Vazi, Keen Negotiator (New Capenna Commander #92)
Voice of Resurgence (Double Masters #227)

The message was clear to Sorin—time to search for something else.