The Riftbreaker

Pretty much do what the description says: build 50 Morpheum Towers. This will come naturally about 2/3 through the DLC but can be unlocked much earlier.

Follow the story until you unlock the tower (really early on). You don’t need 50 at one time and it’s important to note, building a level 1 tower and upgrading it counts as two builds and a third if you upgrade it to level 3 after, so it’s probably worth doing that if you already have the upgrades researched.

Morpheum Towers are powered by the morpheum you pump out of the pools on the Metallic biome maps (so the whole DLC campaign). They can actually be built in the pools, they don’t need power or ammo, just a pipeline connecting them. I believe the achievement simply counts the construction, not having it hooked up so there’s the option of just spamming some somewhere (hold A and scroll to mass build, just like walls).

It’s worth noting that the game naturally prompts you to spam build these during the Morpheum Lake map mission (I might have the name wrong, basically the mission to obtain a crapton of Morpheum and most of the map is a lake and not solid ground). You have little dry land so you pretty much have to build Morpheum Towers by the dozen to defend your base. If you don’t have it by this mission you’ll naturally obtain it there.