Zalgo is a recurring antagonist in a few Slender/ ARG Series.
One Bad Dream
In the series, One Bad Dream, he is the main antagonist. He is described as an unstoppable entity with Satan-like powers and the incarnation of chaos, death, destruction and horror.
Not much is know about Zalgo, but apparently, he was a ancient creature who was locked in an alternate dimension by another entity (probably Am Dhaegar), waiting for someone to release him. Many years later, three boys named: Rob, Dalton and Brandon who begin to experience strange occurrences, realized that they are somehow linked to Zalgo and his loyal follower, The Messenger. He seems to work like a corruptive hive-mind that eats away at a person's sanity and soul. In alternative future show in the series, cannibals, war and chaos are rampant and the government currently pursing Rob and Dalton. Nazis are also linked with Zalgo, during the 1950, Zalgo was almost unleashed on earth by a Nazi Scientist Conspiracy.
Note: Information regarding this iteration of Zalgo does not appear in the Rebooted channel called Project One Bad Dream.