If the domain is a Child domain and if you wish to delete Child domain entries / Trust from the root forest.
We need to Choose “Delete the domain because this server is the last domain controller in the domain”
We received an error while decommissioning the child domain
The Operation Failed because :
Active Directory Domain Services Could not be removed on this Active Directory Domain Controller because this is the last AD DC in the domain, and the domain has a child directory partition
Use ntdsutil to Delete the Child domain
Open the CMD prompt
Domain Management (In 2008 it changes to “partition management”)
Connections => connect to server DC01
List <— to see zones
Delete NC DC=DomainDNSZones,DC=Domain DC=Com (This Deletes the CrossRef Object)
Force replication, validate that the partition is gone.
Restart DNS, the service will re-add the partition.
Now Demote the Last domain in the Child domain. it should allow you to demote without any issues.