He needed certain conditions to enter a human form.
Preferably a normal creature of flesh and blood.
For instance, if Duncan anchored in a divine spirit's realm, it would be difficult for him to complete possession, because the prayers in the divine country are considered private property of a certain divine spirit, and Duncan's attempt to occupy a human form would become very challenging, furthermore constrained by the laws of the divine country itself.
A troubling place indeed.
This means if he cannot capture a human form, Duncan would have to consume Legendary Level, actively find a way to anchor, and even consider seizing some unconventional non-human human forms to enter the real universe.
In low-magic planes, given Duncan's current status, he could operate freely, but high-magic planes are entirely different.
"Let's try auto-capture first."
"If it doesn't work."
"Non-human or even non-natural creature human forms can also be considered."
