Compared to the four remaining Servants, only Rider makes Tokiomi and Kirei the most restless.
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ThatÆs to say, apart from Rider, the four remaining groups of people no longer needed to be spied upon by Assassin.
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To obtain precious information, they can send Assassin to test it out.
Right now, while Rider and his Master were defenselessly having a banquet, itÆs a great opportunity for a surprise assault. At this time, victory doesnÆt matter; the important thing is the difference in battle strength between the enemy and their side. Of course itÆd be best if Rider can be smoothly disposed of; if not, then if they can force him into a desperate situation and get him to use his most powerful ultimate weapon, that would be enough as well.