The Arcane Codex- Slow Zone is one of six codex's which aid the player in their fights or general exploration. Using the Arcane Codex- Slow Zone will summon a 9x9 area centered at the cursor position (so it goes 4 tiles in every direction from where it's "core" is summoned).
This zone specifically slows down hostile NPCs while they are in the zone inflicting them with the Chilly Graspdebuff. The debuff will remain for 3 seconds after the hostile NPC leaves the zone.
Do note that the Arcane Codex- Slow Zone requires a sentry slot to work and that the player is able to summon multiple Zones at once.