Monarch is a member of D'Hoffryn's Magic Council. She is the Fae queen of the woodland sprites, who live for peace, beauty and revelry. It is known that the Fae can't lie and that in her domain people are judged for their character, not their appearance.
After the restoration of magic, Buffy and the other Scoobies created a series of rules to write on the Vampyr book, where each member of the council would have a particular ability that they could use against those who would threaten Earth's balance.
Monarch's ability is to drain the magic power of any being not native to this dimension. However, she was killed by D'Hoffryn, who was planning on using the magic law for himself.