The Ripper (Minoris omniphagea) is a type of Tyranid organism which is essentially a small, worm-like body with a mouth with rows of sharp teeth, and not much else. Hell that latin name up there? It means "Little Eats-everything".
Rippers are generally deployed in the later stages of a planetary invasion, when the Tyranids' hold over a planet is ensured and "tyranoforming" starts in full force. They will eat the planet's native life-forms (and Tyranid casualties, if need be) and once they have gorged themselves they crawl back to the digestion pools and jump in; there, they and the biomass they have consumed get digested, broken down, and later slurped up the capillary towers to the hive-ships above. Note that Rippers lack digestive systems themselves; the swarm is nothing if not efficient, and Rippers are only equipped with the minimal organs required to retrieve biomass and bring it to be processed. Consequently, they rely solely on the Hive Mind to do all the thinking for them and are highly unlikely to survive being seperated from the Hive Mind (e.g. through the death of a Synapse Creature). However, there have been recorded cases of Rippers reacting to the withdrawal of the Hive Mind by burrowing underground, where they use the biomass they've accumulated to metamorphose into just about any other kind Tyranid bioform, from a Hormagaunt to a Hive Tyrant.
In a nutshell, this means that the Tyranids regularly eat their own babies. Or baby siblings. So grimdark.
While small, Rippers can still be a formidable threat to most infantry in large numbers, where they'll drag down and chew the unlucky sods to death. Hell, there's even an instance where even tanks were overwhelmed by massive waves of Rippers that were spawned by the Parasite of Mortrex. The fact that they don't balk at the sight of tanks rolling over them make them an excellent tarpit, provided a synapse creature is nearby to give them directions or the enemy isn't employing massed weapon fire, in which case they'll be nothing more than a minor annoyance.
Rippers are an unusual unit in the Tyranid army since you cannot buy them in a box. Instead they have to be assembled using Termagant, Hormagaunt and Genestealer boxes. The first has 7 Rippers, the second has 4 and the latter has 2. Tyranid Warriors used to have them as well, but with the new Warriors equipped with Bone Swords and Lash Whips the sprue they used to be on has been taken out, so you're stuck with the aforementioned units. But that's okay, since as a Tyranid player you'll be needing a boatload of Gaunts anyway. All you'll need is a bunch of bases and you're good to go. If you're stingy you can get clever with modeling and create a hole in the ground or a bush or something with a Ripper popping out or get creative with those spore things on the Genestealer sprues: this takes more work but you can really get a lot of mileage out of those Rippers of yours this way. Do keep the 50% of the model has to be GW rule and you should be alright.
On the TabletopEdit
Rippers work best in swarms. Coming in units of 3-9, Rippers put you back a mere 11 points each. This does not grant you a very specatcular profile: A4 looks interesting, but at S3 AP0 D1 attacks paired with WS5+ means you'll rarely hit more than 2 attacks per base, and a 3 model unit will hit 4 times on average. And while they have 3 Wounds with a mere T3 and a 6+ save means that they are one of the flimsiest units in the game. For two additional points per model in the unit you can give them Spinemaws, a Pistol 4 that shoots at S2 AP0 D1 at a mere 6" range. This might seem even more pathetic until you realize that this doubles the number of attacks you get at very close range. Do note that at S2 you'll be needing 6+ to even wound a MEQ or anything with T4, so don't expect a lot of mileage out of these guns against Marines. Against the IG or Tau however this is a whole other story, since against T3 you need a 5+ instead. Before you try and take down enemy tanks with 9 Rippers, consider doing the math: 36 attacks at WS5+ means 12 hit, and only 2 wound since you'll need 6+. After that the enemy gets their full save, so if you're lucky you'll land 1 wound. And that's ONLY if you lose no Rippers when you charge.
These little critters do have one edge: they can burrow and pop up at the end of your movement phase more than 9" from any enemies away. This means that you won't get any shots off in your first turn, and you need quite a bit of luck if you want to be able to charge. Slapping a use of Onslaught onto them once they pop up is all but required if you want to get their charge off and fire their Spinemaws. As for Hive Fleet Adaptations, Behemoth and Kraken really help their speed, Leviathan is good for survivability but is situational because of its range, Gorgon is immensely useful to actually wound in combat, using Jormungandr means you messed up, Hydra is useful but situational and Kronos is trash on Rippers.
The best way to use Rippers is to slap Kraken onto them, have them pop up and tie up an enemy ranged unit. This gives them the choice to either break off and forego shooting the next turn (barring certain rule breakers) or try and kill off the Rippers. In either case the Rippers will be subject to a massive amount of small arms fire, which is exactly how you want it: if the Rippers are being shot your large blobs of infantry aren't and you can close in for the kill. If the Rippers aren't killed off and they lose their initial target they can use their speed to get to another infantry unit and try and tie them up. As such, it's best to use Rippers as a single large unit preferribly at max size: for less than 120 points you're getting a bullet sponge of 27 wounds. This might not seem much given that for 3 points more you can get 30 Termagants with Fleshborers, but those don't enjoy the fun being able to Deep Strike in and distract the enemy without a major point increase and are 6 attacks behind.
Note an interesting loophole: as Rippers are Swarms and not Infantry, a lot of rules that would affect most units won't do dick to them. Like a Knight's ability to fall back over infantry, for example. Use this to your advantage.
Three Ripper models clock in at 2 Power, and six at 4. Interestingly 7-9 Rippers cost only 5 Power total, meaning you'll get almost a full power discount on your unit when taking 9 Ripper models with Spinemaws. (Points cost is 11 per swarm model, +2 for spinemaws.)
|Leader Organisms:||Broodlord - Hive Tyrant - Swarmlord - Tyranid Warrior|
|Small Creatures:||Gargoyle - Genestealer - Hormagaunt - Ripper - Termagaunt|
| Biovore - Hive Guard - Lictor - Neurothrope - Pyrovore |
Ravener - Tyrant Guard - Venomthrope - Zoanthrope
|Monstrous Creatures:|| Carnifex - Dimachaeron - Exocrine - Haruspex - Malanthrope |
Maleceptor - Mawloc - Tervigon - Toxicrene - Trygon - Tyrannofex
|Gargantuan Creatures:||Cerebore - Dactylis - Hierodule - Malefactor - Nautiloid - Viragon|
|Flying Creatures:||Harpy - Harridan - Hive Crone|
|Bio-Titans:||Dominatrix - Hierophant - Hydraphant - Viciator|
|Other Organisms:|| Bio-Weapons - Cortex Leech - Mieotic Spore - Mucolid Spore |
Mycetic Spore - Spore Mine - Sporocyst - Tyrannocyte - Zoats