Wondrous item, uncommon
This normal-looking white or yellow flower looks like it grew in a pond as recently as an hour ago. It feels slightly damp to the touch, and none known have grown larger than 4 inches in diameter. It will keep indefinitely in a dry place, but once placed in water the fun begins.
Within 5 minutes of continuous contact with water, croaking frogs start gathering. Slowly frogs start appearing, chirping and grunting, gathering around the lily. After an hour of contact with water, the full magic of the Frog Lily takes effect.
Hopping frogs fill a 10 foot radius around the lily, raising a cacophony of croaking, whistling, and peeping. Spells with verbal components cast from within the frog area require a concentration check (Constitution saving throw DC12); on a failed saving throw the spell fails or comes out wrong (DM's discretion on how the spell goes awry).
The 10 foot radius around the lily becomes difficult terrain and acts like an entangle spell, restraining anyone who fails a Strength saving throw (DC12). The cacophony and entangle effects last for 10 minutes before the frogs start to disperse, hopping off to wherever they came from. It takes another 10 minutes for the frogs to completely disperse, leaving the wilted remains of the Frog Lily crushed by the press of frog bodies.
The frogs will ruin any food or drink in the vicinity. The entire area becomes wet, and any unattended fragile objects have a good chance of breaking. Open scrolls have a particularly hard time surviving this plague of frogs. An area effect spell doing at least 1 damage will kill the frogs and immediately end all the Frog Lily's effects.
Some Frog Lilies summon poison frogs. Some say all the purple flowers will do this, but nobody has confirmed this hypothesis yet. Poison frogs act in all ways as detailed above, but they also cause 1d6 poison damage each round to anyone in the 10 foot radius at the beginning of their turn. A Constitution saving throw DC12 negates the damage for the current round.
Rumor has it that Wyatt Bentreed, the caretaker of Highgarden, uses Frog Lilies to exact vengeance against those who vandalize the garden or hurt his companion animals. Some say he knows the secret of making these nasty little items as well.
Many thanks to @firstname.lastname@example.org for misreading the Mist Rapier's ability as summoning "frog clouds". Even greater thanks for letting me know so this little gem could leap from my mind unbidden.