Samhain was a Celtic witch-god worshiped largely by practitioners of black magic in the Middle Ages. 
For an unspecified amount of time, Samhain observed the Ghostbusters until he was sure of their power and had everything he needed. One June, Egon Spengler and Winston Zeddemore took a break and treated themselves to some Popsicles. Samhain watched them from atop a tree nearby. Egon thought he felt a sinister fall breeze from the park. He decided to take some readings later. Samhain stated Egon would find nothing. He vowed to return for the Ghostbusters later and they wouldn't see him coming.
On June 26, 2016, during the Crossrip Podcast Episode 227, Erik Burnham clarified the details about Samhain in the Tobin's Spirit Guide by Insight Editions would be used in the comic. Samhain was planned to appear in the International arc but there wasn't any more room as the writing went on from 2015 to 2016. Burnham still wants to use him at some point. 
- Ongoing Series
- Ghostbusters Annual 2015
- "A Fall Wind in Summer"
- ↑ PCOC File (2012). IDW Comics- "Ghostbusters Volume 1 Issue #5" (2012) (Comic p.23). Line reads: "After adopting the stage name "Sam Hain" (after "Samhain" -- a Celtic witch-god worshipped largely by practitioners of black magic in the Middle Ages), Hain became a world-renowned illusionist and magician, specializing in death defying escape routines and shocking magic acts (many reports suggest Hain may have dabbled in witchcraft on numerous occasions)."
- ↑ 6/26/16 "#227 - Andrew Shaffer/"Ghosts From Our Past" and Erik Burnham "Tobin's Spirit Guide" - June 27, 2016" 2:29:37-2:30:04 Erik Burnham says: "All the stuff in there, uh, I wrote to work with the comics. So the version of Samhain, for example. The details I put in there. If I were to use Samhain, that's the details I would use. And I, uh, yeah, I did -- I did think at one point last year that we're gonna sneak him into International but not going to be any room for him in International but I still want to use him at some point."
- ↑ erikburnham Tweet 11/29/16
- ↑ Egon Spengler (2013). IDW Comics- "Ghostbusters Volume 2 Issue #9" (2013) (Comic p.16). Egon Spengler says: "They were also used for lanterns in celebration of Halloween and Samhain...in..."