And BG-57 got it right it's Yunalesca!

With several shapes, colors and sizes, three different forms
I'm sure to surprise you, you really don't expect them at first
and from the ancient greeks, a medusa is the final form
you may notice a resemblance.

Beware,
for even though I may seem weak, first forms are freakishly easy
I'm sure to give you hell, hellbiter is one attack and she is literally a pain in the ass to fight
and much, much more... mega death, mindblast and many other deadly attacks...

So BG you're up!