This is a summary on the Team Atlantis Episode starring Demona and Fiona Canmore (Courtesy of Ask Greg:

Milo & Company are in Paris battling a sewer monster. Doc runs into an old flame, Fiona Canmore, who is hunting "a demon". That demon, of course, turns out to be Demona, theoretically the last of her kind. Demona uses a smitten Mole to find the Praying Gargoyle, and uses the statuette to artificially animate every stone gargoyle in Paris -- in order, basically, to murder the local humans. Team Atlantis defeats Demona's plot, but Doc prevents Fiona from killing Demona.

Notes relating to the episode:
All Atlanteans are human, though once upon a time there may have been an Atlantean clan of Gargoyles.

Ingredients of the Preying Gargoyle include an Atlantean Crystal, Gargoyle Skin and some powerful magic mojo.

Fiona Canmore would still be alive in The Gargoyle Universe as of 1995. She's just really old. She was voiced by Sheena Easton. She helped train Jason, Robyn and Jon.