Rules for Inquistors are included in the Agents of the Imperium document here:
https://www.warhammer-community.com/wp-content/uploads/2023/02/kKj593HK6K628syU.pdf
You can include 1 and only 1
Agent of the Imperium unit in any non-
Agents of the Imperium detachment if it is an
Imperium Patrol, Boarding Patrol, Battalion, Brigade and Arks of Omen Detachment. Otherwise, you need to use an
Agent of the Imperium detachment to contain them.
Including an
Agent of the Imperium detachment in your army will almost always reduce the power of the other portion of your army as it will prevent them from gaining a
purity bonus most codexes have requiring all units be of the same faction and sub-faction (Like
Ultramarines Adeptus Astartes).
On the other hand, including a single
Inquisitor in a
Deathwatch Battalion detachment gives you your inquisitor while allowing all possible bonuses to your Marines.