So this is a topic of discussion on some message boards and some in our league have questions with it too:
In the SW codex, the special character on a Thunderwolf is base toughness 5 and so are all the regular Joe Schmoe thunderwolf cav fast attack guys. Not 4(5), but just T: 5. Now, unless for no reason whatsoever all dudes riding thunderwolves in the codex HAPPEN to be tougher than the special characters and as tough as Mephiston, it's obviously a +1 to the BASE toughness from being 2 models on 1 base (guy and mean old mount).
So RAI it's obvious that the base toughness is 5. But what about RAW for ICs other than Canis who start off as base 4 (as do the other guys of course but hey it's RAW) and get the thunderwolf as an upgrade?
Most people seem to think those ICs are now 4(5), because they are misreading the rule on page 26 of the BBB:
"Some models can gain improvements to their toughness value by riding bikes, marks of chaos, etc. When it comes to Instant Death, such bonuses do not count..."
It's important to note that this isn't the same as "ALL models that have an upgrade to their toughness from wargear use the original lower value." It's grammatically possible for SOME models to have a bonus upgrade and still suffer instant death on the lower score, without any rule existing that excludes some unmentioned models from i instance of having a bonus that DOES use the new score for instant death.
In fact, the entire rule is qualified on a certain condition in the following sentence that comes immediately afterwards: "IN THESE CASES..." The rule with using the previous value is ONLY APPLICABLE in instances where two values are listed in the toughness statline (one in brackets and one normal). It says that using a different value for instant death toughness is only in case of where there are two numbers, in that in those cases you then you "use the lowest value for working out instant death."
Now, obviously characters that upgrade to bikes or marks of nurgle don't have this because a bike/mark is not in the base stateline because it's an upgrade to the model. So TECHNICALLY a rules lawyer would be correct in saying that the lower-value instant death doesn't apply to these models based on how page 26 is worded. It's against RAI since the bikers or plague marines or whatever are 4(5) but RAW the rules lawyer would be right.
But in both of those cases there's a second rule that kicks in to apply to models that buy bikes or the mark as an upgrade. It EXPLICITLY and SPECIFICALLY says under the bike entry in the BBB as well as the entry under the mark of nurgle in the chaos codex, in seperate sentence/rule, that even without the x(y) thing from page 26, you STILL nonetheless use the original toughness pre-upgrade anway.
Anyone who will glance on page 62 of the Space Wolf Codex will notice that this caveat is glaringly missing. Models that are upgraded to a thunderwolf mount simply "add +1 toughness to the profile" with any of the two possible qualifiers that would apply the lower-value toughness for Instant Death on the model.
Therefore thunderwolf mount upgraded ICs are base toughness 5 and can only suffer Instant Death from str 10, since there is nothing that would force it to use any other value. (They are also str 10 with power fists but that is obvious).
Reading carefully for the win!
