Malphas

Malphas is the true secondary antagonist of Dead Of Summer. It was a demon that was banished to the lake of what would become Camp Stillwater by a pastor named Holyoke.

Backstory
However, there was a cult that worshiped him and remained in area. The cultists killed Holyoke's congregation and made it appear as though he was the cult leader, causing an angry mob to lynch Holyoke. The cult however remained in existence.

Present Day
After Deb Carpenter reopens Camp Stillwater, she hires teenagers to be counselors. Malphas notices the arrival of a possible vessel into the world, and orders his henchmen to begin the ritual to free him from the lake. They remove the heart from a deer and bury it in a designated location to draw an inverted pentagram over the camp. This gives Malphas enough power to cause the counselors to be haunted by visions from their past. Malphas also has enough strenght to (partially) appear to one of the counselors, Amy, as she would be the perfect vessel because (unknown to the viewers, and the other counselors) she was a complete psychopath who killed her family and her best friend. Amy wants to use this power to kill more people, and agrees to help the Malphas. She starts by killing the janitor and using his body to continue the pentagram. She then kills Blotter and Cricket to keep the pattern going. Malphas' cult believe Amy to have been hypnotized (even though it was of her own free will) and "kidnap" her. Amy draws some of her blood and drops it into the water.

Malphas orders his henchmen to kill themselves by slitting their throats. The pentagram is finally completed after they die. Malphas' return is marked by a solar eclipse, during which he gives Amy some of his powers. However, there was no eclipse scheduled for that day, leading Jessi, Joel, and Garrett to realize that the ritual was successful. Cricket appears to Jessi and tells her to throw Holyoke's bones into the lake, which she does. This triggers a flashback that reveals what really happened with Holyoke, and they realize that he was just another victim of the cult. Later that night, Joel is killed by Amy, who they take to a cabin to exorcise.

Deb loads the kids onto the bus to bring them home, but Malphas prevents them from leaving by coating the bus with blood. When the driver gets out to investigate, he is killed by Malphas' presence. Meanwhile, Malphas attempts to free Amy several times, and succeeds in getting Jessi to open the door by appearing to them as Deb, which taints the pure lake-water that was being used during the exorcism. They believe Amy to have been exorcised by Jessi, who nearly drowned. When Amy gives Jessi CPR, she commands him to come back to her, possessing her completely this time.

Later, Amy/Malphas kills Deb and Krissy. After realizing that Holyoke's soul is in Jessi, which will prevent Amy, who has Malphas' power from leaving, they zombify the Amy's victims. Amy kills Garrett and the police who arrive to investigate. Amy, with Malphas' power disguises her voice to trick Jessi and Alex into thinking the police are there, and that they are safe. After they come outside Amy/Malphas kills Alex, but is trapped in a cabin with the last bit of pure lake-water. Jessi plays Holyoke's sacred hymn over a walkie-talkie, which kills Malphas when he listens to it.

Trivia

 * Malphas is the bigger bad to his cult, although Amy is the bigger bad to Malphas.
 * Everyone believed that Malphas had possessed Amy because she was an innocent, however Amy was actually possessed because she was evil, and was a serial killer before she even arrived to camp.