The Night Spinner is the Eldar indirect fire ordnance vehicle. It's based on the Falcon chassis and mounts two bigger versions of the monofilament guns used by Warp Spiders called a Doom Weaver. Before they weren't that great...
Now they are the fucking kings of horde clearing.
For a time the fire prisms outdid them in the fact that they had an AP value with their big plates but now the Night Spinners have an AP6 to their nasty S7 (which will ALWAYS wound on a 2+ unless you're fighting other eldar because you wound comparing strength with initiative, which means you'll have it easy even when dealing with MCs). This means their intended targets (ork boys, gaunts) will get mulched 5/6 times. But wait! What about cover saves!? Oh boy, their weapon has two modes: one large blast that is barrage (read: you don't give half a mon'keigh fuck about intervening terrain and models' cover) at 48 and the big winner: a TEMPLATE (read: you ignore cover AND cause D6 hits on the poor sods that ride the open topped transports-so you rape Deldar and Orks) that has fucking torrent. Remember how chaos players love the burning brand? We can almost spam them, as this vehicles can be squadroned. And should.
Why? Because unlike the fire prisms their special 'combined shots' don't sacrifice one or two shots. YES, you can shoot 3 S9 templates at 20 away from the enemy. This means it's feasible to go and instagib ork mobs, nobs and T4 models (something the dispersed fireprism can't do thanks to its measly S5). Hordes will die against this, guys.
Oh my, someone forgot about the cake's icing! It has monofilament: to-wounds of 6 become AP2. This thing can be used to punish TEQs. On a 6 man squad, 18 hits (that won't scatter) will cause 3 AP2 wounds and 12 AP6 wounds. That means a whopping 4 dead TEQs. Look at a fire prism squadron: they'll cause 18 hits and cause 15 ap2 wounds meaning a whole 10 man TEQ squad dead... IF they don't scatter out of the squad and IF the termies aren't spread out. Being realistic, we are looking at 8-9 hits at best and 6 wounds that will result in as many TEQs as the Nightspinners should they be out of cover, so they could cause even less if they had fallen into ruins.
In fact, the nightspinners work better than them against things that rely more on cover saves than armor. Not only boys but also many marine units such as squads that can crank 3+ or even 2+ saves with the right tactics (like the scout formation). Against 6-man devastators with 4+ cover saves, they'll wound 3 AP2 times and 12 normal ones, resulting in a whopping 7 dead scouts. The dispersed modes, should they not scatter, will cause 12 wounds that will result in 6 dead devastators.
In recent years
8th edition wasn't kind to the Night Spinner. The loss of templates virtually erased the tank's role as an anti-horde vehicle, as a fairly random 2D6 hits makes it rather unlikely (if not borderline impossible) to wipe a 10-man squad, much less anything more numerous. It's not all doom and gloom though. It does retain the ability to bombard targets outside line of sight, so it does remain a viable artillery piece for a terrain heavy map. Additionally, a flat 2 damage per successful wound and the potential AP-4 gives it a solid anti-MEQ/TEQ niche since it'll effectively one-shot most Primaris marines and Terminators who fail any saves they can make.
9th edition so far is slightly better for the tank. The addition of Blast weaponry gives the Night Spinner considerably more consistency against infantry squads. The ability to move and fire the Doom Weaver weapon without penalty is also nothing to scoff at either. Having said that, the Night Spinner's very conditional AP and the Blast rule doing nothing when the weapon in question targets units infantry squads of 5 or less tends to make it somewhat underwhelming against MEQ in general for the cost to field it.