Instead of just a normal Dagger of Venom I was thinking that someone found (and used) a Khalkos stinger as the base/special material for the Dagger of Venom.
Everyone killed with this stinger probably got infected with brain larvae, but since all the victims so far were burned none of it will spread.
But if someone from the group should take this dagger and not burn their victims they might create the starting point for a khalkoi invasion.