To describe a feeling or atmosphere:
* Eerily: This is the most direct option, but can sound a bit stilted.
* Uneasily: This conveys a sense of unease and discomfort.
* Uncannily: This suggests something strange and unexpected.
* Creepily: This emphasizes a feeling of unease and dread.
* Unnaturally: This suggests something that is not normal or expected.
To describe a sound or sight:
* Eerie: This can be used as an adverb here, but it can also be used as an adjective.
* Ghostly: This suggests something spectral or otherworldly.
* Hauntingly: This emphasizes a feeling of sadness or melancholy.
* Disturbingly: This emphasizes the unsettling nature of the sound or sight.
Ultimately, the best adverb to use will depend on the specific context and the effect you want to achieve.