Witch Particle Effect is one of the Cosmetic features on Minecraft Central. It was added in 25th April 2015 and belongs to the category Particle Effects. It works across the whole server.
The only way to obtain this feature is by purchasing it separately from the server web store. It can be bought individually for $3.99 USD or part of the Bundle pack for $19.99 USD. The effect can be enabled through the commands /Effect and /Effects or the Cosmetic Menu in the Lobbies.