The Siren sister from earlier episode makes another appearance in chains at Surt's feet. she forces Slimer to attack Eduardo, and rip off his protective gear.
Banshee and Syren returns from Sonic Youth.
Useful Fact: The original Ghostbusters had also dealed with the concept of Ragnarok in the RGB episode Ragnarok And Roll.
Surt had been imprisoned by the original Ghostbusters, but there is no time frame given for that battle. I conjecture that that battle happened "off camera" as part of the bigger battle in "Ragnarok and Roll".