The Heart of Kandrakar

The Heart of Kandrakar originally is a  white crystal pendant in a silver frame with a few carvings. While it's usually shown with a bright glow, on rare occasions it just appears as a normal pendant with something like a transparent glass marble in the middle.

But the Heart can have other shapes and take on different colours. For example instead of a marble, it can appear in the form of an auramere that's hanging from a different version of the silver frame. Sometimes there's a chain attached to it, other times not.
Sometimes there's no frame at all and the colour varries from dark violet, bright pink, light blue, white to a dull grey. The colour is partially determined by its current keeper, but also by things like the unity of the guardian team.

The location the Heart appears from is also different from time to time. It can for example glow and emerged from the chest of its keeper or from its hand. By closing the hand, it disappears again. And it can of cause be carried like a piece of jewellery, like Yan Lin always uses it.

Not many keepers of the Heart are known. The ones we definitely know are Will Vandom and Nerissa. After Nerissa's fall to darkness, Cassidy received the Heart and after her death it was passed on to Yan Lin. Before C.h.y.k.n. however it is not sure who carried the Heart.
Will Vandom surrenders it to Matt while dreaming (#75), which kind of makes him a temporary keeper. He takes it away by order of the Oracle Himerish, to prepare the girls for the arrival of their new pure elemental powers. After that the Heart reappears to save Yan Lin (#86) after she's been severely injured in the attempt to restore the Anchor of the fortress. It's said it had lost power, possibly due to the elemental pure powers to be taken by WITCH while it was parted from its keeper. But through Yan Lin's actions as new Oracle, the Heart gained power again and healed Yan Lin in return. It later greets Will and seems happy to see her but remains around Yan Lin's neck. That indicates that the Heart either has no real keeper at the moment and simply is back in the fortress, or that it chose Yan Lin as its current keeper.

Apparently, if a keeper dies, a "spark" of the Heart remains as a part of their soul. It is this that Will received from Cassidy in order to defeat Nerissa.

Also, the nature of the keeper determines the kind of Heart. After Nerissa's new appearance, she taints the Heart with her dark mind and unleashes its pure darkest potential.

From what is known in the comics, the Heart can only be taken, if the keeper gives it up freely. So for example it is being passed on to another person. If it is taken by force, it has to be an immensely powerful magic accomplishing this and it only means the Heart was ripped from the keeper's hands, not that it can be wielded by another. Since the Heart also has a conscience, it chooses its keeper. If the person is not deemed worthy, the Heart refuses and fights against the keeper.

Even if lost or passed on, the Heart always keeps a special bond with each keeper. It always returns to its keeper one way or another. It remembers them, you could say.

Over time the Heart has consumed many different matters which might influence its "mind" and "actions". Here is a chronological list of foreign matter the Heart of Kandrakar has absorbed over time.

The Seal of Phobos - is absorbed together with Phobos' book and all black matter that had come out of it. The seal enables W.i.t.c.h. to open portals and travel to Meridian.

Nerissa's Heart - is actually the real Heart of Kandrakar. She tricked Will into giving it to her out of free will (Issue 20). The Heart then turns dark, breaking apart and releasing the "negative essence of the Heart of Kandrakar. "Evil in its purest form". This dark essence originated in Nerissa herself and hence is what I would call a "foreign matter". It splurges all over the fortress until the Heart melts with Cassidy's star and the rain washes the last bit of the dark fluids away.

The Spark of Cassidy's Heart - Just like Nerissa's darkness that she called upon in the original Heart, the spark of Cassidy is also a foreign matter. Through merging the two pendants, a piece of Cassidy's soul itself becomes part of the Heart of Kandrakar. While it's not mentioned later on, it seems to be Cassidy who makes the final decision of the keeper, Nerissa or Will. Nerissa even tries talking to her in an attempt to convince her she'd be the better choice.

The Heart's Shadow - Ragorlang

The Light of Meridian - After Elyon's imprisonment by Oracle Endarno (who really is Phobos at that point), she entrusts Yan Lin with her very magical essence, the Light of Meridian. Yan Lin delivers it to Will who keeps it in the Heart of Kandrakar and later uses it to be able to regain the Crown of Light from Meridian.

The Heart of Kandrakar has many, many functions and uses. For one it can absorb magical matter if it is the dominant force (which - in the end - it always is). It also allows the girls to summon their powers and become guardians, in other words: make them transform. It can conjure different kinds of magic to free or capture forms of life (e.g.: the Banshee) and is also able to heal it's bearer. It can transport or teleport people across the universe. It can shrink (enemies for example) and create (astral drops) and holds a variation of magical barriers W.i.t.c.h. can use. It enhances the powers of the guardians and supports them through guidance and actual assistance in combat. It is even able to break mighty spells or repel them. It can find portals and communicate directly with its keeper by showing images in the glass marble.

Cornelia doesn't trust the Heart at the beginning of the comics. It doesn't please her that Will lets the Heart decide what, when and how to do things. She also expresses her concern that the Heart might not work someday, which would put them in danger. Will on the other hand holds great trust in the Heart and almost relies a little too much on the Heart saving them or making the decision instead of giving it a try herself.

Generally the Heart seems to have a conscience of its own. Several times the Heart makes own decisions. It chooses its owner, it chooses the next step the girls should take, it chooses to transport them to Kandrakar. It seems that the only way to become a good keeper of the Heart is to become "friends" with it. If the bearer is liked by the heart it heals her or helps her with the many tasks. If not or if the keeper seems mentally unstable or uncertain it hurts her and the burden becomes heavier.
In the special "The Year Before" the narration is done by the Heart which again supports the idea of it having some kind of conscience.
The heart enters a kind of relationship with its keeper, not only from an emotional point of view but also when it comes to strength. The Heart gives strength and heals, but it can also hurt and feed on power. Nerissa's Heart and the shadow of the Heart show that there definitely is something dark about this powerful object.

In the end it's probably a question of circumstances and use.

Each time the Heart encounters a foreign matter similar to itself, especially if it's another Heart like the spark of Cassidy or the shadow of the Heart, a small fight of domination seems to take place. This fight determines which matter is the stronger and engulfs the other. The Heart of Kandrakar practically always wins. Still it means that there can only be one Heart at a time. Sooner or later, even several Hearts will melt together again. Once come to existence, a new Heart will "call" out to the other and once the Hearts come close to each other, the stronger will unite with the weaker and sort of "feed" on its energy. The winning Heart also gains powers from the matter it united with. For example Cassidy's spirit became part of the Heart of Kandrakar, once it and her spark had melted together. Another example is the ability to open portals to the Metaworld after the Heart sucked in Phobos' seal and book.