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If my Night Goblin Great Shaman uses an arachnarok as his mount, does he count as part of the crew? The rules say that you can't hit the crew in close combat. So, can the shaman be picked out in combat or not?
The shaman is not part of the crew and can be directly targetted in close-combat.
See the rules for 'ridden monsters' on pg105 of the rulebook.
Also night goblins can't ride the araknorok just normal goblins