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I wanted to use this formation alongside my reclamation legion, but I don't know if it exists anymore as I can't find Retrobution Phalanx in my 2015 Necron codex.

The Retribution Phalanx formation comes with the Start Collecting! Necrons box set.

Like other formations it can be used alongside the rest of any Battle Forged or Unbound army.

 Mr. Shine wrote:
The Retribution Phalanx formation comes with the Start Collecting! Necrons box set.

Which was created well after the codex in question, I might add.

So you can use it alongside the Reclamation Legion, but keep in mind it doesn't count toward anything for the Decurion detachment, if that's your intent. It's a legal formation, and it can be run with the Legion, but it doesn't count as a core, auxiliary, or command choice to make a Decurion.