Also known as "Zombie Essence" because of its effect.
The infected vial allows a player to play as a Zombie. Unlike normal essence which are obtained via killing monsters however this one may only be obtained by being "cannibalized" by another zombie player. It may only be obtained once on each character slot but it can also be sold to other players. (Usually for around 1mil-2mil)
When an Infected Vial is obtained the player will emit a strange greenish glow indicating that they have received it. Level, race, and class do not seem to affect the drop rate.
The animation when obtaining an infected vial
Generally people farm for Infected Vials on the world map. Though it is impossible to cannibalize players in dungeons, it is still possible to cannibalize them in CTF, thus Infected Vials can technically be gained there as well, even if the player is cannibalize
Someone receiving an Infected Vial after being cannibalized by their own teammate, in CTF
d by a team mate.