Ultra Beast Lillie, also known as Daughter Beast Lillie, is a fusion of Lillie and Nihilego.  The fusion has caused Lillie's eyes to turn a dull beige/yellow color and her personality has become completely corrupt by Nihilego, bending every bit of Lillie's mind to its will.  This allows Nihilego to speak through Lillie, since Lillie herself is entirely under Nihilego's control.  Daughter Beast Lillie adopts a much darker personality, very similar to Mother Beast Lusamine.