I read an excellent post by NidsandWolves over on Facing The Grey Tide
about this topic. I strongly suggest you read it as there's a really useful rundown of how each unit will be affected by Instinctive Behaviour. This post will contain similar information but hopefully help me get my head around this part of the Tyranid codex so I can make some informed conclusions about it all. I know I've covered it briefly but I'd like to look at it in more depth to really decide how it affects the army.
Synapse Creatures & Synapse Range
Obviously the first thing to think about with Instinctive Behaviour is how to avoid it (well that's the first thing I think about). There are a handful of units with the Synapse Creature rule which can be employed to prevent the army collapsing due to Instinctive Behaviour. Here's a list of our options:
- Hive Tyrant***
- Tyranid Prime*
- Trygon Prime*
Fearless and Auto-Regroup
There's at least one option in every FOC slot then. These units start with a basic synapse range of 12" and are themselves Fearless. There are two reliable ways to extend Synapse Range with Norn Crown and the Dominion psychic power adding 6" range each (and as far as I can tell these stack). I've indicated next to each one what options they have available "*" indicates just Norn Crown, "**" indicates just Dominion and "***" indicates both. That leaves Warriors and Shrikes who are stuck with 12" range no matter what. In addition the Synaptic Lynchpin warlord trait increases the basic range of the warlord to 18". The Swarmlord comes with this by default but Tyrants, Primes and Tervigons (unless they're Troops) will need to roll for it. Unless you're taking Swarmy then you can't rely on it.
Potentially then a Hive Tyrant or Tervigon could have a 30" synapse range if they succesfully cast Dominion and get the warlord trait. Bear in mind that you roll for warlord traits BEFORE psychic powers which means you wouldn't need to necessarily swap your random power for the Primaris (Dominion) if you've already got an 18" basic range from the warlord trait.
Personally I think 40pts is a bit steep for a 6" boost to Synapse. Had it been 12" then I think the price tag would be more reasonable. To me, Norn Crown may as well be a big fat target painted on the model equipped with it. Kill the Norn Crown model and a whole swathe of Tyranids will behave irratically. Additionally you might not always want to take Dominion on something like a Flyrant as the other powers are, for the most part, extremely useful. The Swarmlord may as well always take it though as he's WC3 and will therefore get three powers. Since Tervigons will nearly always be surrounded by gaunts I think it's a given that they should always have Dominion too.
Keeping the Synapse Creature Alive
With the interweb up in arms about how easy it is to kill Synapse Creatures and wreak havoc with a Tyranid general's plans, I'm sure every man and his dog knows they should be a priority target. How then, can a Tyranid player avoid losing their precious units so easily?
Some of the Synapse Creatures are more durable than others. A single zoanthrope won't last long even with a 3++ save so you'd better have a back up. Tervigons make a great choice as they're both Synapse and can kick out those extra gaunts. As I've said, it's worth taking Dominion to prevent losing one of the Tervigons being an issue. Since blessing powers fall in that conveniently grey area in the "start of the movement phase" you could theoretically extend your range before checking which units need tests.
It's worth noting that Synapse doesn't require line of sight which means you can happily stick a Zoanthrope in a Bastion and place it in the middle of the table. Remember Bastions don't need to be placed in your deployment zone but rather in your table half. That means you stick the Zoanthrope inside on turn one and you've covered a huge area. Of course in the middle of the board it'll be easy prey for a melta or two but it's worth considering.
At the end of the day the usual 40K tenements apply, if something is worth having it's worth taking multiples. The same applies for Synapse Creatures. You'll want some redundancy built in. Most lists are likely to feature a couple of Tyrants, Tervigons and maybe even a Zoanthrope or two. That gives you decent coverage of the board. The problem with the Tyrants is that they'll be wanting to get stuck in and are going to be priority targets regardless. Tyranid players are going to have to get used to the possibility that they'll lose their Synapse Creatures and be forced to roll for Instintive Behaviour but how much of an issue is it?
Before I talk about the effects of IB on individual units I wanted to point that Fearless is a HUGE bonus. Those 4pt termagants can hold up pretty much anything in combat that doesn't have many attacks. That can be a real force multiplier if used well. I think there's a lot of attention paid to IB rather than focussing on the benefits that Synapse brings.
Of course, without the Synapse Creatures the tyranid army will be weaker but whilst they're around the whole army is going to be pretty tough to shift. To me it comes down to distraction but I'll talk more about that in my summary post.
The Effects of Instinctive Behaviour
Here's a quick run-down of which units are affected by which branch of IB, I've listed their leadership in brackets:
Lurk (Deathleaper (10), Termagants (6), Lictors (10), Venomthropes (6))
Fail your Ld test and there's a 50% chance the unit will Fall Back. This obviously isn't good news for a unit that's already lost Fearless. The other half of the time they'll be forced to look for some area terrain to jump in to let them function, there's a chance they'll get Stealth, but they'll still not be charging. This obviously isn't good news but remember they aren't force to do anything half of the time. Venomthropes are likely to already be in cover of some kind and aren't likely to be far away from the Synapse Creatures in the Tyranid deployment zone. Lictors and Deathleaper aren't likely to be in Synapse range but with Ld10 they'll only suffer from IB 1/12th of the time which is hardly worth bothering about and let's face it there aren't many Lictors around anyway.
Termagants suffer the most from this as, with their poor leadership they're quite likely to get IB and aren't likely to regroup should they roll "Survive". It's worth considering sticking your objectives in area terrain to minimise the damage should you roll "Seek Cover". At least then they'll still be useful.
Feed - Tyrant Guard (7), Old One Eye (8), Hormagaunts (6), Rippers (5), Haruspex (7), Pyrovores (6), Raveners (6), The Red Terror (8), Sky Slashers (5), Hive Crone (10), Carnifex (7), Mawloc (8)
By far the most common IB, Feed has possibly the worst effects too. There's a 50:50 chance that a unit will attack itself and given that it's resolved at the unit's strength that means 4+ to wound most of the time so a hefty number of casualties for units with 6+ saves. Looking through the list of units that suffer it, Tyrant Guard are likely to have died before the Tyrant they're guarding. Old One Eye will be safely on a shelf with the Rippers and Raveners. No-one will worry about Sky Slashers or Pyrovores either. Single models don't suffer Cannibalistic Hunger so that rules out Mawlocs (who can simply burrow), Crones (who are Ld10 anyway) and the Haruspex.
The problem with Feed is that it can prevent Dakkafexes from shooting and could force Hormagaunts to charge and leave an objective behind. Realistically both will probably just end up either killing themselves off slowly or doing nothing. Hormagaunts will probably be your meatshield anyway so might've died before any of the Synapse Creatures start to fall.
Hunt - Hive Guard (7), Gargoyles (6), Harpy (10), Biovores (8), Trygons (8), Exocrine (7), Tyrannofex (8)
It's worth noting that the Harpy, Exocrine, Trygon and Tyrannofex are all Fearless which means no "Burrow and Hide" for them. Prowl isn't great for a lot of the artillery units though who'll want to shoot distant targets most of the time.
Finally, there's genestealers who don't suffer from IB at all. Now, I'm not suggesting that makes them worth taking but you could potentially hide some in cover to control an objective reliably without worrying about their proximity to the rest of your army.
The units which are hardest hit by IB are mostly gaunts. Most other units either have decent leadership or won't suffer too badly from the effects of IB. Let's not forget you have to fail a leadership test to suffer the effects and most of the units are Ld7 or above. For example at Ld8 you'll pass it 72% of the time even Ld7 passes 58% of the time.
I know there have been plenty of battle reports on-line where the tyranids completely folded because the synapse got taken out but I really don't think this would be the norm. A lot of the time you'd have enough other synapse creatures to fill the gap and even if you don't the majority of your army isn't too badly effected.
Clearly the main issue is your scoring units, after all that's what wins games. If you can't keep them in Synapse then chances are they'll struggle to stay on objectives. With your Synapse Creatures alive though they'll be fearless and tough to shift. I'll talk more in my round-up but for the most part I don't think IB is as big of a deal as people are making out. It sounds really bad on paper but it isn't that difficult to avoid and the pay off of having a Fearless army is worth it in my opinion.