I don't get abyss drop only mods.

Hi, I wanna start build making in 0.5. I really enjoy mana stacking in poe 1, so I want to attempt it in poe 2. On craft of excile I saw this mod (4-5) spell damage per 100 maximum mana, which is a kurgal abbys mod. My understanding is that you omen>kurgal mod/prefix and then reveal, but never encounter this mod while revealing. When you click the mod in the calculator portion of COE it says "drop only mod."

This whole adventure leads to the following questions. Where does it drop form? Can I add it to a staff without garbage mods? Why can't I find an article on how these drops work?

This question left me in a pickle since encountering this mod. Please help :)