Rules question: When you take control of an enemy warjack or warbeast, does it become part of your battlegroup while under your control? Here's what I can find in the Prime Digest Rules:
p. 61 "If you take control of one of your opponent’s models, it is friendly for as long as it is under your control."
pg. 68 "If a rule causes a ’jack to become an enemy model, it is not part of its original battlegroup while that rule is in effect." So it's temporarily not part of the opponent's battlegroup.
p. 68 "Battlegroup Commander ... This model and the ’jacks it controls are collectively referred to as a battlegroup."
The incident was when my opponent playing Iron Mother used Domination to control my Slayer under his feat turn. We wanted to know if the feat increased the Slayer's MAT to 8. A tournament-going player said no, so we went with that, but I'd like to find the actual rule.
Related question: does the controlled model temporarily lose buffs previously placed on it because it is no longer friendly?
Oh, here's another situation I thought up: I'm playing Skarre1 into another Cryx player and I've got Dark Guidance up (Dark Guidance: "While in the spellcaster's control range, friendly Faction models gain an additional die on their melee attack rolls this turn."). If I then use a machine wraith or warwitch siren or Cephalyx overlord to control one of my opponent's models, does it gain the benefit of Dark Guidance because it's Cryx and temporarily friendly?
There's a problem with the use of the word "control" being overloaded. Jacks your caster controls are your battlegroup, but all models in your list are under your control (as the player). When you take control of an enemy warjack, it's you the player taking control of it, not your warcaster. The problem with looking for a rule that says it doesn't become part of your battlegroup is that instead there's no rule that says it does.
I think @dogganmguest has it right: "control" doesn't mean the same thing in those sentences. Also, I'm pretty sure the FOC = MAT/RAT explicitly only works on Vectors, so the Slayer wouldn't go up regardless. That does raise an interesting question about what happens when she takes control of an enemy Vector though.
Dark Guidance is an entirely different case. As your first post noted, when you take control of something, it becomes friendly. The Slayer is of the Cryx faction and so is the list Skarre is in. Both of those together make the Slayer friendly faction and thus affected by Dark Guidance.
PS: lol, I can't tag you because the forum filter changes the text to dogganm.
So "take control" refers to the player not the warcaster. And spells that say something like "While in the caster's control range, friendly faction models gain..." do apply. Got it. Thanks!
What about spells worded like "Target warjack in the spellcaster's battlegroup gains..."? Does the spell effect continue to work because the jack has already gained the effect, or does the spell effect not apply while the jack isn't part of the caster's battlegroup?
"Target warjack in the caster's battlegroup" is a phrase that means to cast the spell, your caster has to have LOS to an applicable model (that is, your caster must target that kind of model). Since the controlled jack isn't in your battlegroup, it's not a valid target for the spell. Also, that's at the time of casting, and I don't think there's any effect that lets you take control of something long enough to cast another spell on it.
"Target warjack in the caster's battlegroup" is a phrase that means to cast the spell, your caster has to have LOS to an applicable model (that is, your caster must target that kind of model).
Sorry, the situation I meant to ask about was if my warcaster casts something on their own battlegroup warjack and then my opponent takes control over the jack. Was a bit unclear. But if the phrase indicates when the caster can cast the spell, then it sounds like the "gains" effect stays active while my opponent takes control.
Oh, my fault, I didn't read as thoroughly as I should. Yes, spell effects that originally targeted a battlegroup model can still apply when being controlled. It can depend on the wording of the spell, but generally once something gains an ability, it retains it while being controlled. Also, models in a unit affected by a model/unit spell lose the effect when being controlled because they aren't part of the unit.