pf2e.fyi
From Pathfinder Battlecry!, which isn't enabled.

Preserve the Horde

FEAT 10
ARCHETYPECONCENTRATE
Activity free action
Archetype Necrologist
Prerequisites Necrologist Dedication
Trigger You cast summon undead.
Requirements Your horde is raised.

The act of summoning additional undead minions doesn't make you lose the focus required to maintain the animation of your horde. You Sustain your horde.

Pathfinder Battlecry!ORC