The glaive does not work against the Beast. Lyssa (Lysette Anthony) and Prince Colwyn (Ken Marshall) have to work together to produce fire from water. Colwyn then hurls that fire at the Beast, destroying it. The pair along with Torquil (Alun Armstrong), Ergo (David Battley) and Titch (Graham McGrath) escape the Black Fortress before it disintegrates.
See this movie for the cast which includes a young Liam Neeson, a young Robbie Coltrane, a young Francesca Annis, an old Freddie Jones and a James Horner score that he had to have lifted for The Wrath of Khan!
Thanks Evil Ed!