Melusine is an old and powerful ekhidna living in a large complex of caves near the village of Svorlag, located in Skellige. It is presumed that a cult once worshipped Melusine as evidenced by the large winged statue located in the caves beneath the mountains which she inhabits.
I... I saw a shadow... Great wings, like a cloud passin' o'er the sky. Then I heard him scream. – Britt of Svorlag
In the mountains near the village of Svorlag in Skellige lies an enormous complex of caves. For many centuries they were given wide berth on account of the bloodthirsty ekhidna known as Melusine who had made them into her lair.
Melusine was a beast so powerful some islanders worshiped her as a semi-divine being. The witcher, however, harbored no such delusions. He knew she was an extremely powerful monster - but a monster all the same. This meant she could be killed. To do so, he would need the highest quality bolts, a solid silver blade - and a steady hand.