If you use erase from diskutil my understanding is that file recovery is still possible, as unlikely that a buyer would attempt such a recovery. If this is not a problem read no further.
If you want to securely erase everything your best bet is to do a full init from the Promise Utility or CLI.
Assuming that you have the original configuration the CLI command would be...
init -a start -l 0
Be sure to disable the MAC sleep as the Pegasus will power off when the MAC sleeps.
A full init of a logical drive can take a long time. But if you do want to use it, after it's done you can use the macOS Diskutil to create a new filesystem on it.
A quicker way to scramble things would be to power off the Pegasus, re-arrange all the disks, then power on and delete the logical drive then the disk array. Then use the Promise Utility Wizard automatic option to create a new filesystem. The data will still be there on disk, but it will be out of order and unrecognizable.