Player 1 has the tainted with miasmatic presence (MP) active and wins a heroic move (HM) roll off. Then on his HC move the tainted charges the enemy line. Now the MP 6" covers more of the battle line before the other player get to do his HM move. The question is does the MP (as a passive that lasts for the remainder of the turn) only affect models in range when the HM were called at the start or when the HM are actually taking place? I would assume the later and that one would draw the 6" after player 1 is done with his/her move as the second player has not yet been able to start his HM nor call a with me. Any thoughts?

Your conclusion is correct. The effect of MP, like other non-instant aura effects (banners, an aura of command, stand fasts, and so forth), is calculated based on the distance between the model generating the effect and the model being effected at the time the model being effected activates. In your example, the proper order of action is for the second HM to take effect; models in range of the calling hero to be checked; and then models in range of the Tainted to be checked. Those models that are in range of both the calling hero and the Tainted would not be able to take part in the heroic move.

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