Guild Wars 2

Did they have they own stats which can be increased only by traits or stats scale from necromancer stats?

Their stats don’t scale with yours(which is a real shame). The only way to increase their power is through specific traits, like the tier two spite trait that increases their damage by 30% or the tier two death trait that boosts their HP by 50%.

As Stx said , you can only increase their damage +30% and HP +50% via traits.
But they also benefit from boons such as fury and might.

The Demonic Spirit.3157:

As Stx said , you can only increase their damage +30% and HP +50% via traits.
But they also benefit from boons such as fury and might.

so it’s actually doesn’t matter which gear use?

Exactly
Traits and boons are the only ways

The exception being any conditions they apply, which scale with your condition duration and (for damaging conditions) your condition damage.

Bhawb.7408:

The exception being any conditions they apply, which scale with your condition duration and (for damaging conditions) your condition damage.

!!
Minions don’t apply condition damage ( don’t mention death nova as it’s not a minion’s skill )

On their own, no. But via combo fields and specific buffs they get access to conditions, which all still apply your condition damage. For example death nova procs a poison field, and bone fiend fires through it. Bone fiend has just gotten access to poison, and while he is the one applying the poison, it is your condition damage that determines the damage.

Also, Jagged Horrors inflict Bleeding with their attacks. Condition damage applies.