Nightspinner rules are not in the book, you can find them here
Basic rules are:
Statline - same as falcon (AV 12 12 10, transport 6, BS3)
Gun: Twin linked (reroll scatter dice), large blast, Barrage (doesn't need LoS, but if no LoS, you don't subtract BS from scatter - not that important with a TL gun)
Gun: Strength 6, rending, Monofilament web**
** Monofilament web: any units hit by this blast take a difficult AND dangerous terrain test if they try to move the next movement phase.
I have 2 Fire Prisms, and find they work best in pairs - takign only one usually means it'll blow up before it does any damage (or gets disabled). 2 gives you the ability to combine shots, which means S10 AP1 small blast(anti-landraider), or S6 AP3 large (nasty against marines)
EDIT: Nightspinner is not mentioned as transport in that rule set, but it's old. Nightspinner is mentioned in one of the transports sections in the 6th edition BRB, so I assumed it's capacity of 6, since it's the same chasis as a Falcon/Fire Prism. Not sure how that holds up in tournaments.