Exspheres

From Aselia, the Tales wiki

Exspheres are very important in the plot of Tales of Symphonia. Exspheres serve as power-enhancers for the party and the antagonists, the Desians and ultimately Cruxis.

Originally, Exspheres were thought by the main characters to be simply objects that increased the user's fighting and survival potential far beyond his or her maximum limits. However, it is learned in Kvar's Human Ranch that Exspheres are made by Desians by cultivating humans and turning them into Exspheres. This revelation almost caused Lloyd to discard his Exsphere, but Kratos convinced him otherwise.

An Exsphere acts like a parasite in its host and is life threatening without a Key Crest. If removed without a Key Crest the host's mana goes haywire and deteriorates the host until he/she becomes a monster.

The party later finds out that Exspheres were not just in Sylvarant, but in Tethe'alla as well. Most Exspheres in that world were taken from the Toize Valley Mine, though their origins were the same. In fact, it is possible to trigger a cutscene within the game where Tabatha, Altessa's doll, takes Presea to the Exsphere mining site and shows her how beautiful it is. Most people with Exspheres in Tethe'alla were given them by the Renegades or Vharley, an Exsphere broker.

Exspheres also were supposed to be somewhat of a way to turn humans into elves. It's a lower level of power compared to the Cruxis Crystal though.

[edit] Tales of Symphonia: Dawn of the New World

Although in the end of the first game, Lloyd and his friends intend to work together to collect and dispose of the remaining Exspheres in the world, several characters appear to still be wearing theirs in Tales of Symphonia: Dawn of the New World (most likely to serve as a reminder of what Exspheres are capable of).