You cannot ally with an army from your own codex. Otherwise you would already have Salamanders + Iron Hands lists, or Iron Warriors + Emperors Children, or a mix of Tau cepts, guard regiments, Necron Dynasties or any number of other mixes etc.
Gonna have to wait and see how it works with any rules in the supplement though.
I am the Hammer. I am the right hand of my Emperor. I am the tip of His spear, I am the gauntlet about His fist. I am the woes of daemonkind. I am the Hammer.
Now that the codex is out, please move your conversations to the appropriate 40K subforums.
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