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This is a Grand Illusion question... Can the Grand Illusion be used to place a unit into Reserve that was on the table and vice versa? It states that you can deploy said unit following the deployment rules. I read this as yes... you can but to me reserve is a deployment as well as placing any unit on the table.
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Interesting, ive never come across this question before. Of the top of my head id say no... but maybe someone else will be able to give a better insight, 40K rulings arent my speciality.
I see it as you do, reserves are a deployment option. Ive always played it that way and haven't had any opponents question it or complain.
A couple times Ive placed a Monolith on the table to affect my opponents deployment and then GI it to reserves to deep strike. Also I use it to deploy a heavy to push back their deployment and then GI it to a better position. Both throw a major wrench into their game plan and can give you a big advantage before 1st move is even taken.
And over there we have the labyrinth guards.
One always lies, one always tells the truth, and one stabs people who ask tricky questions.