The Halloween series consists of songs uploaded on or close to Halloween each year from 2008 to 2012.
Each song (except for Halloween Monster Party Night) has the numbers "1031" somewhere in the song. This is read as "Halloween". Most of the songs also have "lulilu lilala" sung, sometimes more than once. It is usually mentioned as a "spell" or "cursed words".
Much like the last song, it is sung by a group of monsters having a party surrounded by the dead and other monsters. At the end "you" utter the spell (lulilu lilala) and are turned into a monster yourself.
This song started the 1031 and luliu lilala pattern.
A song about a pumpkin monster. This is the only song to be sung by a single Vocaloid. It does not have "lulilu lilala" in the song, but it does have 1031.
The backstory of the Puppeteer in Darker Halloween YumYummyGillia, it's about his love for his doll Veldea and Veldea feeling guilt over not being able to keep the promise hey made.