Lady Kersgreth, Skeleton Witch

Whenever a Skeleton creature enters the
battlefield under your control, you may pay {b}. If you do, put a +1/+1 counter on it.
{b}, {t}: regenerate target creature
{1}{b}, {t}: create a 1/1 black Skeleton creature.
If Lady Kersgret should die, put her onto the
bottom of her owner's library instead.