Fluff and Rules
According to the fluff, Tyranid bioforms such as Warriors, Hormagaunts, Termagants, and Gargoyles are essentially more advanced and/or specialized forms of Rippers. The Hive Mind can further enhance these forms to monstrous proportions: e.g., Trygons are essentially enormous Raveners (which are themselves a variation of Warriors), Harpies are big Gargoyles, etc. The new models are no exception. According to the recent White Dwarf, the Maleceptor is an "adaptation of the Zoanthrope", while the Toxicrene is an "expression of the 'family tree' begun with the creation of the Venomthrope".
|Imagine these guys, but big enough to actually need limbs|
Both have the standard Tyranid WS and BS, as well as the same toughness as most monstrous creatures. The Maleceptor's strength is one point higher than the Toxicrene's, while both have five Wounds. The Maleceptor, being a Synapse Creature, has a higher Leadership. Sadly, both have a 4+ save, meaning that they won't get armor saves against the majority of weapons suited for monster hunting. A 5++ on the Maleceptor and Shrouded on the Toxicrene improve their survivability, though.
The Maleceptor is essentially a psychic assassin with a pretty mean focussed witchfire attack. This power, and the big bug's mediocre close combat ability, mean that it's designed to stand back and pick off high value targets. The cost of the unit seems too high for what it does, although I don't have a whole lot of experience with 7th Edition Tyranids. Regardless of its abilities, I find the model to be pretty boring.
It's the Toxicrene that really interests me. This bug has a very short range Poisoned (2+) large blast attack that can actually damage a vehicle that has lost one or more hull points. Where it really excels, though, is in close combat. The Toxicrene has six Poisoned (2+) attacks that strike at I6 thanks to its lash whips. Being a monstrous creature, all of its close combat attacks are AP2. On top of that, it has the Hypertoxic rule, which means that To Wound rolls of 6 have the Instant Death special rule. GW threw in Toxic Miasma and Acid Blood for kicks and giggles.
Although the Toxicrene model shares most of the same parts as the Maleceptor, the model is a lot more interesting to me. The lash whips and the tentacles around the mouth make an enormous difference.
Although I appreciate that GW is continuing to develop new models and rules, this release is downright odd. Tyranids got their most recent codex in January 2014, making theirs the second to last full codex of 6th Edition. (I don't count Legion of the Damned or Imperial Knights given that they aren't really meant to work as stand-alone armies.) Now, a mere nine months after getting a new codex, Tyranids have two entirely new units.
In recent years, the only new units added to an army outside of a codex release were the Stormtalon and the Ork flyer variants. The reason for this was obvious; when 40K 6th Edition introduced Flyers, the new models ensured that Space Marines and Orks would have Flyers along with those armies that had models grandfathered into the Flyer category (i.e., the Stormraven, the Razorwing, the Doom Scythe/Night Scythe, and the Valkyrie).
But the Toxicrene and Maleceptor aren't new unit types at all, they're just monstrous creatures with new abilities. So why were they released less than a year after their army's latest codex?
Third Party Paranoia
We're all aware of GW's newfound phobia of third party model makers and the crusade to eliminate any pre-existing units that don't have a model or to avoid introducing rules for a new unit unless they're accompanied by a kit. It's possible that GW wanted to release the Toxicrene and the Maleceptor earlier this year, but didn't want to introduce even more models at the time of the Tyranids' 6th Edition update. Or perhaps their manufacturing capability wasn't sufficient to simultaneously produce another large kit. Rather than risk creating new units for 6th Edition and allowing third parties to produce models of them, they simply decided to release the rules only after the models were ready.
Appealing to the Hobbyists
Big models seem to be the thing right now, with Imperial Knights, Wraithknights, and the new giant Fantasy models selling pretty well. Maybe GW believes that there's a critical mass of large models that will get hobbyists interested in starting a new army that can be filled with them. Two and a half years ago, I was strongly tempted to start a Tyranid army dominated with monstrous creatures. I have to admit that these new models may have re-sparked my interest.
Filling a Gap
The Tyranid codex may have been one of the most disappointing codexes of 6th Edition. After their mediocre 5th Edition codex, Tyranid players were hoping for a competitive boost. The new codex wasn't exactly what they were looking for. (Although some players believe the Rising Leviathan dataslates released afterward significantly improved the army.)
While the new units look interesting, opinion on their rules has been mixed. No one seems to be able to figure out how they're supposed to improve the army or what gap they're supposed to be filling. If GW believes that these new models fill a niche in the army, I have yet to see any guesses on what it could be.
The fact that GW is releasing models outside of the codex is a good sign, though, since it suggests that they may become more flexible in addressing armies' weaknesses in between codex releases. Maybe they'll finally release a model for the Mycetic spore along with a re-release (or an improved version) of its rules.
Preparation for a New Codex?
Not long ago, I suggested that GW would update the 6th Edition codexes to match the style and power levels of the 7th Edition codexes. I guessed that this would happen within three years of the last update of a codex. With these new models, I wonder if GW is testing the Forge World method: release a new model with experimental rules, allow the gaming community to play test it for you, adjust the rules as necessary, and publish the rules in an updated book.
Come to think of it, it's strange that GW hasn't been doing that all along.