19 cards where the illustration contains “hidden-monster”

Acidic Slime (Secret Lair Drop #134)
Arixmethes, Slumbering Isle (Commander 2018 #38)
Beloved Princess (Throne of Eldraine #7)
Cabal Coffers (Modern Horizons 2 #325)
Dandân (Arabian Nights #12)
Dark Depths (Coldsnap #145)
Glacial Crevasses (Ice Age #187)
Hidden Horror (Portal Second Age #75)
Hunting Grounds (Judgment #138)
Hurkyl's Recall (Antiquities #10)
Locked in the Cemetery (Innistrad: Midnight Hunt #60)
Lurking Predators (Magic 2010 #190)
Marit Lage's Slumber (Modern Horizons #56)
Summoning Trap (Zendikar #184)
Thing in the Ice // Awoken Horror (Shadows over Innistrad #92)
Thing in the Ice // Awoken Horror (Shadows over Innistrad #92)
Twilight Mire (Eventide #180)
Wanderbrine Rootcutters (Shadowmoor #178)
Wolf's Quarry (Throne of Eldraine #184)
Worms of the Earth (The Dark #56)

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