According to Greek mythology, Charon is the one and only "ferryman of hell", and tasked to bring souls into hell. While in Grimgrimmoire, Charon acts as a means of transport for familiars under Necromancy, by taking them in it's boat. While it can carry a number of familiars simultaneously, they are to be in astral form, thus it converts non-astral familiars into astrals before letting them take the ride. Charon cannot carry massive familiars like dragons or chimeras. Same as the Obelisk, it acts a supportig role during battle, being unable to launch attacks.
Charon should be counterred with Homunculus's Psychic storm before it is able to drop off its troops. One they have established a beachead within the player's base, the consequences would be devastating.