A rough overview of the boot sequence on a modern computer running a UNIX like operating system is as follows:
The last step is what is needed to be configured for a goetia based init to be started.
If the Kernel or initramfs use the defaults,
/sbin/init is used as init.
Symlink system-init to /sbin/init
to have goetia started on init.
Usually, this executable can be found at
/bin/system-init or /command/system-init.
If a custom initramfs is used,
the end of its scripted sequence
executes into init.
Change the script to point to system-init.
Initramfs source files are usually found at
/usr/src/initramfs.
It is alternatively possible to
change the Kernel command line parameter
init=/path/to/system-init.
This can be done during compile time of the Kernel
or in the boot manager
(e.g. GNU GRUB's GRUB_CMDLINE_LINUX=init=/path/to/system-init).
The last possibility is to change the compiled in kernel default,
e.g. in make menuconfig at
General setup -> Default init path
or by changing DEFAULT_INIT in the .config file.
Open the file /boot/loader.conf.
Change the line saying init_path="/sbin/init:/rescue/init"
(the part after the = may vary)
to init_path="/path/to/system-init:/sbin/init:/rescue/init"