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So my question is pretty simple. How does the reanimation protocol rule function in a kill team battle where each model acts as an individual unit
I couldn't say with any real certainty, but I would have to assume you would not get RP.
Models with RP require a unit to come back to (unless they're Ever Living), so where each model is it's own unit, there would be no unit to come back to; thus no RP rolls.
Unless of course, you houserule that for Kill Team games, Necrons can have their RP rolls regardless. It's not that big a deal, RP rolls fail more often that they succeed anyway.
"Peace, through superior firepower."
perhaps houserule it to either RP on 6 instead of 5+, or RP on 5+ if another model of the same type is alive within ?" (ofcourse then comes the question of string deaths)
X X X X X L (L = living ,X = dead) would you roll for the X:es from right to left, stopping if the reminders are out of ?"
Anyhow, need a houserule as necrons pay for the ability
By the rules, no, they don't get rp. In killteam, they become a unit of 1, and the last model to fall in a unit does not get rp.
But, to fit in with killteam, I would create a houserule that if a model falls within 6" (or 12", whichever suits you) of a like model, then it would roll for rp. Basically, something similar to the old WBB rolls of the last codex.
RP is an ability that is included into the cost of the model. By not allowing RP rolls, you're overpaying for a subpar model.
RAW: This is up to interpretation. Kill Teams say "each [model] is treated as an individual unit" and "all models operate as individual units in this mission". On one side, you could say this means they are units unto themselves and thus RP doesn't work. On the other side, you could say that "treated as" and "operate as" does not mean the model actually is; although precedent says so, the argument remains that models were bought together (such as a 5-man unit) and RP works within that "unit" even if the models act individually.
RAI: As mentioned, the 4th edition Battle Missions didn't take into account 5th edition rules, and thus the new Codex shouldn't be penalized. I think it's a safe assumption to say that models bought together, even if treated as individual units, still support each other for RP. So, if you make a team out of 5 Immortals, 5 Warriors, and 2 Tomb Blades, then like models will get RP. The only contention might be, does the model have to be nearby? RP focuses on groups in unit coherency (2"), so perhaps that same distance is required as well.
As it was said before, you don't get it, but lets think of a fair rule that will help.
I think that the rule that if a necron infanty model is within 6" it should get a WWB save because the cost is included, and it isn't going so far as a single warrior that is really lucky is gunning down the whole army.