how do i unlock the second mysterium incantation? for my weapon

[Kaidalen]

what title says

[Otets Oleksii]

You have to fulfill the weapon's condition and find the Second Mysterium Incantation itself. If you are asking where to find it, threats like miniboss or the Watcher at the Scarlet Coast always drops it. The Watcher is the easiest, if you have the Fallen Star demonic weapon.
Alternatively, you can "research" one incantation at a time via mirror once you've reached Gnosis 1, but I highly not recommend it.