Wanda can examine each of the piece of the altar. Most of the dark energy is gone. What remains appears to be residual rather than active. As Wanda and Alucard search, Wanda will feel the presence of Soris, who appears to recognize the altar from a time long ago. She will encourage Wanda to look for a plain symbol carved on one of the pieces. It'll be faded and ancient, and quite difficult to make out.

Once the piece is located, Wanda can choose to offer some of her magic to Soris to learn more or she and Alucard can capture a few critters on the island to sacrifice instead. The island is, after all, overrun with vermin. Some balance should be restored to please Soris.