The Red Skulls is a gang that seem to have an untold amount of control over the Sharn neighborhood known as Dura. Monster Squad encountered some of their members in Game 3 when they were trying to pass through town, when some Red Skull thugs attempted to mug Monster Squad. The fight ended when the Squad knocked out the goons and arrested them.
Other Red Skulls were seen in Lower Menthis during Game 8, when Monster Squad was trying to investigate the crime scene of the hostel attacked by the Sharn Stomper. During this encounter, the gang didn’t attack the Watch, and were apparently uninterested in the investigation the Squad was conducting. As of game 14, it is unknown why they were patrolling in the first place.
Apparently, Monster Squad’s encounter with them in Game 3 was an oddity, since before then they were not known to have violent encounters with the Watch. This is likely because the Sharn Watch stays out of Dura for the most part.
Not much else is known about the Red Skulls. Their numbers, power, and the name of its leader are all unknown by Monster Squad and the Sharn Watch.