"trick and treat" is OSTER project's first Rin and Len duet song. It was uploaded for Halloween 2008. OSTER project's works are usually cheerful-sounding, but "trick and treat" was her first work to have a darker atmosphere. The song reached one million views on November 21, 2011, making it the second Kagamine duet song ("Gekokujou" being the first) to do so.
"trick and treat" basically tells a story of "you", who was led by Rin and Len into a deep, deep forest. Although they seemed friendly at first, "you" realized things were not as they seemed.
A fanmade PV that is a bit less dark and with a more cute approach by 7:24, and probably the only one with a "happy end". Miku get lost in the woods and runs into Rin and Len. They approach her with deceitful intentions that end being more childish that it seems in the beginning.
On October 1, 2012 it was announced that along with Ichika and Kurousa-P's Cantarella manga adaptation, trick and treat would also receive a manga adaptation on Nico Nico Age Premium Vol. 15; illustrated by NEGI. Chapter 1 was released on October 9, 2012 for 85 yen.