Zalgo is essentially what happens when you combine a Lovecraftian Eldritch Abomination
with modern-day Internet Memetic Mutation
and a dash of meta sensibility. Its origins are two strip hacks, one of Nancy wherein she invokes him
to get out of "walking Aunt Fritzi's turds," turning her and a boy with her into monsters, and another of Archie
whrerein Mr. Lodge tells him and Veronica
to be wary of Zalgo and the end times, with a final panel of his pupils dilated, against a black background, screaming "ZALGO!"
The idea behind Zalgo is that he/she/it represents corruption. Corruption of innocence, corruption of normality, corruption of the world as we know it, the list goes on but you get the idea. Zalgo is the embodiment of that which is right decaying grotesquely into something very wrong.
The internet meme is to have corrupted text indicating his approach. We won't burden you with the font here. This site
will let you easily Zalgo-ify text. If your browser has trouble rendering the Zalgo text, try here
Tropes associated with Zalgo: