I have just come out of the Serpent Temple after killing Elyssa.
I have retrieved the Mask of Evil and also her Stick. What do I
do with them? Do I keep them or Sell them??

Her staff is most likely cursed.
But the Masque.. the masque is very important. You'll need it later in the game.

Ditto Nanobyte's response. You will need the Masque in your next dungeon (or two). It also makes a nice helm... [img]smile.gif[/img]

Most magic staves (aside from the witches' sticks) are cursed and not worth using, IMHO.

1. The staff is cursed surely. Otherwise, it's a normal item. You can sell it, use it (but don't forget it's cursed), or throw it away.
2. The mask is a quest item needed later in the game. It's the "Face of Death" talked about by the Oracles. You will need the mask in the Shurugeon Ruins.