Hexxus is a demon of smoke and is the main villain in Ferngully, not experienced by the Spongebob gang but by the Pooh's adventures gang.
Hexxus is the spirit of destruction and all that is toxic to nature. As such, his only goal is to cause destruction, death and decay in anyway he can.
Hexxus first appeared on Earth in ancient times when the balance of nature shifted. He rose out of the Earth and rained down poison, destroying much in his wake and killing countless living creatures, including humans. He finally was defeated by the fairy priestess Magi Lune and imprisoned inside an enchanted baobab tree, which he remained trapped in for hundreds, if not thousands or possibly even millions of years.
The movie FernGully's main male protagonist Zak's superiors cut down the enchanted tree that Zak had unwittingly painted an X mark on marking it to be cut down. Unfortunately, this turns out to be the tree Hexxus is trapped in, and the cutting down and processing of the tree by the mobile lumbermill known as "The Leveller" releases him. Determined to get revenge upon the fairies of Ferngully (specifically Magi Lune) for imprisoning him, seeing the useful potential of the human machine, Hexxus decides to take control of it and use it to exact his plan. He manipulates the lumberjacks and their tree leveler into tearing down the rainforest in the direction of the fairies by the next morning, thereby destroying the very fabric of their world. He does this under the vocal guise of their boss.
Sometime latter, upon reaching Ferngully after having successfully destroyed much of the rainforest, Hexxus reveals himself. With Magi gone(having sacrificed herself to give her power to all the other fairies), it is up to Crysta and the other fairies to defeat him. He and the leveler chase the fairies and drive them into their home tree despite the several valiant efforts of Crysta, Zak and Batty to stop him. Hexxus scares the lumberjacks off and takes control of the machine. Just as Hexxus is about to kill off all the fairies, Zak turns off the leveler, causing Hexxus to lose the energy he was gaining from it and disappear, but he soon rips out of the machine as a giant demonic skeleton creature made of oil that breathes fire. Crysta seemingly sacrifices herself like Magi did by allowing Hexxus to devour her in one bite, and uses her powers from inside him to grow a seed she had with her to again imprison Hexxus (together with the leveler) inside a new tree with help from the fairies led by Pips. In the last moments, Hexxus can only watch in horror as the enclosing roots seal, thereby imprisoning him again, and hopefully forever.
Role in the Series
But in reality, Hexxus did not stay sealed away in his new tree prison for long. As it turns out, Hexxus had been one of the original Dark Spawn Lords that had been in Chernabog's Inner Circle of Demons, and one of the few who remained loyal to him even after the Dark Spawn where first banished. Despite this, Hexxus was able to find a way to escape Hell, and enact the events of Ferngully's past, leading up to his final imprisonment in the new tree and supposed demise. However, not long after Hexxus was sealed again, Malefor found out about the smoke god's past, then dragged him back to the Banished Realms in order to redesignate him for a second chance, since Hexxus was in fact a spirit of darkness and was also an obscure villain, something that Malefor finds desirable for requirements in order to be a Dark Spawn Lord. His desgination is that of pollution, toxins, and (obviously) destruction. Hexxus was briefly freed during the Black Cauldron crisis, but was unable to do anything except laugh before getting sealed away once agian in the Black Cauldron. In SpongeBob and Friends meet Hercules, it is revealed that the Titans (Lythos, Hydros, Pyros, and Stratos) are Hexxus' sons.