The highest reward from the Hostile Merges event is the Glaxion Vandal which improves on the original in almost every single way.
Better base critical chance, status chance, reload speed, damage, reload speed and magazine capacity. If that was not enough we also get a small area of effect at the end of the beam. The range seems to be three meters but its full functionality eludes me and the damage it can deal seems inconsistent, especially for targets directly behind the initial one.
Other than the regular "mandatory" mods I would highly encourage the use of fire-rate with something like Vile Acceleration or Primed Shred that has the additional benefit of punch-through. This is because the magazine supports sustained high fire-rates without the worry of ammunition as the ammo capacity was also increased.
Overall the Galxion Vandal is an outstanding weapon, fully capable of shredding through high-level targets and while it falls just a bit short of the titans it does have a riven disposition of five out five. I would highly recommend this primary to just about anyone. Enjoy!