Seven Voyages of Zylarthen Supplement 1: Book of Spells includes three spells that annoyingly refer to Supplement 2: Book of Fiends. I say "annoyingly" because Book of Fiends has not yet been published (It's written, but this time I'm pursuing a different option for the art):
Phantom Stalker: This spell summons a Phantom Stalker. See Supplement 2: Book of Fiends for characteristics and effects. Magic-Users. Level: 5.
Spectral Hound: This spell summons a Spectral Hound to track opponents. See Supplement 2: Book of Fiends. Magic-Users. Level: 4.
Twinling: See Supplement 2: Book of Fiends. Magic-Users. Level: 6.
Not a lot to go on there, obviously. So here are those creatures (two days ago I also added them as an addendum to the Electronic Edition PDF):
Read the descriptions as well as some commentary on Fiend Folio and Book of Fiendshere.