I did main mission so fast and there is no Vortex Horror

[Nep]

Need to kill at least 25 , how can I find them?

[Red Ducky]

They spawn all over the map. Just walk around.

[Emphoise]

There are a ton of them in the "anomalous cave" (part of the main quest), you can easily complete the achievement there.

They spawn alongside phantoms. Sometimes they start spawning after you kill a number of phantoms.

You can find unlimited frequent phantom and vortex horror spawns in the crater around the scaled citadel. Just run around

[gizmo6023]

It seems that once you have finished the main quest the spawn rate is reduced but they are still around. I first thought that completing the main quest closed the link between realities and shut them out but later got a surprise when I got ambushed by a group of several phantoms and two vortex horrors.

[Emphoise]

Originally posted by gizmo6023:

It seems that once you have finished the main quest the spawn rate is reduced

I don't think so. It's still hard to move from an object to another in the crater without being attacked by phantoms.

I don't think the spawn rate is reduced at random POIs as well. I'm still seeing phantoms everywhere.