The Avatar of Disease can be found in Antonica just west of the Wizard spire to Tenebrous Tangle. He is a necromancer class and as such will cast mostly disease based spells plus poison spells. At the start of battle he has a necromancer pet by his side which may vary from fighter to scout to dps caster class to plausibly a healer class.

As a necromancer you will have to watch out for life tap/life drain abilities that he will use to recharge his health with. It can also be assumed that he will summon minions/adds to his side to defend himself. A bit of a rumor is that during the fight he will change his shape/form which will cause different effects during battle.

Like all avatars he has a spell that increases his health plus some physical avoidance and/or physical mitigation abilities. His level of difficulty is currently unknown, but is considered to be one of the tougher avatar mobs.