"虚幻的乐境" can be translated as "illusory realm of bliss" or "fantastical paradise", depending on the context and intended nuance:
"Illusory realm of bliss" emphasizes the deceptive or unreal nature of a seemingly joyful or idealized state.
"Fantastical paradise" conveys a whimsical or imaginative utopia that may not be grounded in reality.
Choose the phrase that best fits the tone and meaning of your original text.