玻璃啼魚-グラス・マーメイド-

You musn't make a sound.
The mermaids' throats haven't yet evolved the means to allow both breathing and speaking at the same time. If you make a sound, you'll lose your means of breathing...and drown.
Keep this promise, and I shall grant you legs for a single night. If you can successfully engage in "sexual relations" with a land-dweller, then you shall remain a human forever—such is the magic.

The price? It's fine, the payment can be deferred. In any case, the illusion only lasts for a single night. A mere instant. It'll be over soon, right?