Lore of Tzeentch attribute query

I think this is covered in the general principles.

Quoted page 10 brb
To make a characteristic test, roll a D6 and compare the score to the relevant characteristic in the model's profile. If the score is equal to or less than the number in the profile, the test is passed with no ill effect. If the score is greater than the number in the model's profile, the test has been failed, and something nasty will occur, as detailed in the rule that called for the test. Where a model (or a unit) has more than one value for the same characteristic, as is the case with cavalry, for example, a characteristic test is always taken against the highest of the values.


So a unit of goblins with a black orcs big boss, test on the BOBB's T5


Edit: adding in the combined unit's rule, noting characteristics tests (aside from leadership) is not an exception.

COMBINED UNITS Whilst a character is part of a unit, both he and the unit (including any other characters that have joined that unit) are treated as a single combined unit for all rules purposes, save for the exceptions listed here.

Cheers gents

The same applies to ridden monsters. Usually, they can use the highest value, but if hit by a template spell requiring a test, rider and mount test separately and, depending on the wording, may be required to use their own value. 

Although not specifically stated, it follows from the general rule. Normally, you cannot hit rider or mount separately, but in the case of templates BRB p. 105 applies: "If you use a weapon or spell that uses a template against a monster mount, both the riders and the mount are automatically hit if the monster's base is touched by the template (note that this includes bouncing cannonballs!).

The fact that Purple Sun hits both and requires a test for both, already indicates that, in this case, they are not counted as a single, but separate models. Since Purple Sun hits models, not units, each model takes the test using its own value.