Evil talismans

The Evil Talismans are talismans that Evil forms/ grows. They hold the power of a villain. Whether this is only when they are stripped of their power like what happened to all the villains that got imprisoned on the Isle of the Lost or not is not entirely clear. Fact is that Maleficent seems to only have power when holding her Dragon's Eye Scepter that contains her Evil Talisman, a (green) Dragon's Egg as aye. Though in the live-action movie Maleficent this is not the case: here she can use magic also without the scepter, which suggests that the talisman only increases her power. This is in line with the fact that Mal can use magic without a Talisman, though you could argue that her magic is not really evil in nature.

Background
Yen Sid talks about these Evil Talismans and their relation to Evil and the Endless Catacombs of Doom in Return to the Isle of the Lost:

"Over the course of the twenty years since the dome was created, magic grew wild and flourished underground, where it created a system of tunnels, the Endless Catacombs of Doom, which compose a series of magical lands underneath ours. Some say these tunnels also include an escape route out of the Isle of the Lost and straight into Auradon itself.”

"Evil is a real thing, it lives and breathes, it works its malice through living vessels eager to spread its vile wickedness, but villains cannot be villains without the source of their power. Every villain has a talisman. These talismans hold the powers that were stripped from them upon their exile to the Isle of the Lost."

Return to the Isle of the Lost
Yen Sid explains where the parents of the original VK's are:

"The Catacombs of Doom are only one part of the equation. When Maleficent escaped (in Descendants), she released so much energy into the surrounding area that it sparked a magical reaction underground and caused the recent earthquakes and unseasonal weather phenomena all over Auradon. She’s also awakened four evil talismans that have been growing in the Catacombs of Doom over the years. These talismans are growing in power, and causing havoc with our weather as they draw energy to themselves. Four of these talismans are the most dangerous right now: the Fruit of Venom, the Golden Cobra, the Ring of Envy, and a new Dragon’s Egg. The talismans desire to be found; they have already seduced their masters (respectively Evil Queen, Jafar, Cruella De Vil and Maleficent (not that she can do this, as that she is an imprisoned lizard at the moment)) into looking for them and soon they will call others to their side. They seek to escape the darkness of the underground so they can once again bring chaos and catastrophe to our world above."

Then he tells the VK's that they must find and disarm the talismans before they fall into the wrong hands and then bring them back to Auradon, where they can be destroyed forever.

After this he tasks his Anti-Heroes club to find the entrance to the Catacombs that is somewhere on the isle. They find it and the original VK's go down there to find them.

The Catacombs of Doom and it's Talismans
Every talisman has created it's own magical land in which it can be found, if the seeker accomplishes the challenge the talisman provides.

Fruit of Venom
The Evil Queens talisman...