A quick Googling seems to indicate that the siyokoy is a Philippine monster not too dissimilar from the eponymous Creature from the Black Lagoon. So Pathfinder’s eel-like version admittedly may not be canon…but it’s still pretty cool.
I always like medium-to-high CR monsters that can be deployed without a lot of flipping through the spell lists or reading special ability mouseprint. The siyokoy is just such a tussler—it’s a CR 10 damage dealer with a stunning electrical tail slap and over 100 hit points on its side, end of story.
(Okay, wait, so this story has an epilogue: Also, note the DR 10/slashing. If you’ve got a GM who’s strict about underwater combat rules (where slashing weapons tend to stumble), you’d best have a freedom of movement option handy.)
Why else do I love this monster? Two subraces included right in the description with differing temperaments for PCs to encounter. Plus the eelfolk have an archaeological obsession with the sunken ships and cities of surface dwellers, paired with a striking taboo against dwelling among the ruins (and doesn’t that just reek of portent!). There are lots of eeeeevils below the waves, but not a lot of bona fide mysteries. Siyokoys provide the latter in spades.
Exploring a sunken city, a party of adventurers stumbles upon siyokoy artifact hunters. The reef siyokoys hunt them during daylight but retreat at nightfall, for they hold the ruins taboo after dark. This may be the prudent strategy—the moonlight reveals streets that were not previously there and second-floor colonnades that vanish by dawn. Also with dawn comes the return of the siyokoys, now bringing a bed of reinforcements.
Chancellor Mur’gelik Senge is a bloated hulk of a deep siyokoy whose vast, gulper eel-like belly often hangs heavy with the corpse of his latest meal—sometimes one of his rivals. To his people, he is the only thing standing between them and destruction at the tentacles of their great kraken overlord. In reality there is no kraken—the “chancellor” uses their fear to keep them docile and excavating a temple decorated with dragons this world has never seen.
The siyokoys on Y’tenerth are a created race. When the Severed Oracles pulled a comet from the sky and sunk half of Unjar, the shogunate district of Moaka appealed to the empyreal lords to be saved. They were turned into eelfolk and escaped the majority of the cataclysm. Centuries under the waves have slowly diminished their memories and their love for the agathions who saved them, but they still have a fascination with the sunken remnants of the empire they left behind. This fascination does not extend to a tolerance for surface dwellers who might be after the Severed Oracles’ prognosticating magic.
—Pathfinder Bestiary 3 246
I saw Creature from the Black Lagoon in 3-D in 5th grade. For anyone growing up in Columbia, MD, media teacher, free-outdoor-movie shower, and all-around evangelist for film Mr. B. was/is an icon.
Speaking of free outdoor movies, I forgot to mention that yesterday I caught the last 45 minutes of The NeverEnding Story at a free showing last night. They still look like good, strong hands…