Monarch was a member of D'Hoffryn's Magic Council. She was the Fae queen of the woodland sprites, who lived for peace, beauty and revelry. It was known that the Fae were incapable of deception and that in her domain people were 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 was 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.