The Vanguards of Fenris are three souls enslaved by Fenris and turned into monsters.
According to legend, in order to pave the way for Ragnarok, Fenris must enslave three souls to be his vanguard, one each of the elements air, land, and sea. Only then will the destroyer known as Surt will appear. Fenris accomplishes this by freezing a chosen one. They then slowly transform into a creature. Egon Spengler became the Vanguard of Air, Garrett Miller became the Vanguard of Sea, and Roland Jackson became Vanguard of Land. Since the Ghostbusters challenged him at the Museum of Natural History earlier, Fenris chose these three to be his. Unwilling to risk hurting the trio, the Proton Guns were set to stun. The Vanguards returned to normal after Fenris was flung into the Containment Unit by Kylie.
- In some interpretations of Ragnarok, Fenris appears with four other figures (not three like in this episode): Hel, Liki, Garm, and Jormungandr. Jormungandr was said to be cast out into the sea. These figures might have been the inspiration for the vanguards.
- Egon Spengler (1997). Extreme Ghostbusters- Slimer's Sacrifice (1997) (DVD ts. 06:40-06:46). Sony Pictures Home Entertainment. Egon says: "According to legend, he must enslave three souls in order to fulfill the apocalyptic prophecy."
- Kylie Griffin (1997). Extreme Ghostbusters- Slimer's Sacrifice (1997) (DVD ts. 09:56-10:04). Sony Pictures Home Entertainment. Kylie says: "It says that if Fenris acquires three souls to be his vanguards; one for the air, one for the land, and one for the sea, then there will be no way to stop the arrival of the destroyer of Surt."
- Kylie Griffin (1997). Extreme Ghostbusters- Slimer's Sacrifice (1997) (DVD ts. 11:25-11:27). Sony Pictures Home Entertainment. Kylie says: "The Vanguard of the Air."