This main villian, Malvern, is a person who was forcefully Infused with Water during the Apocalypse and is somehow able to siphon off the Earth and Air elements from people to balance out the Water. It is the Water connection that allows this element vampirism. He is a nearly unique creature, a darker side of Baird, who was Infused with all elements in balance.
The name Malvern could mean "an evil defense" or an "evil warding off." Etymology says "vern" in proto-Germanic languages meant such things. That is too perfect for this villian.
Malvern is an Apothecary the characters meet early on. He is helping with the chaos, providing medical support. He appears to be normal on the surface with potions of middling strength, but perhaps odd combinations which an experienced apothecary would question and even consider genius. His true and secret specialties are in sleep potions so he can consume victims' Earth and/or Air elements. Sometimes he needs one of them, sometimes both.
When things really kick off though is when Malvern learns of the unique mosses (or just gets a sample from the party if they collected it) they found in the Shadow Dungeon. When properly distilled and brewed into a potion, they allow a dissolving of the conscious mind and a lapse into the Spirit World. He has some mishaps (perhaps there are strange reports of people dying in their sleep or reporting insane dreams or even spirit/Chaos possession), but eventually learns via an evil spirit, Balon, who he captures and can summon in battle (Water Shaman finds out through his spirit guides that an interloper has recently come and Balon is gone), to sequester his soul in the spirit realm. If his body is destroyed he is able to ambush and possess others who enter the Spirit Realm (via deep dreaming {meet someone who mentions having vivid dreams who eventually is taken} or actual travel) and take their bodies. The only way to destroy him is in a two-part battle: first destroy his physical form and then immediately enter the Spirit World and follow his essence (find a cool way to do this- maybe you need something of his that he owned before the Apocalypse, something imprinted with his true essence). Once there, you must defeat his Spirit Form and blast his soul with pure Fire to destroy it (Lightning is not enough) or take it with you and find a way to purify it.