Female nullification is a catch-all phrase for a number of procedures. At a minimum, it will include a clitoral removal. It usually also includes a hood removal as well as complete labial removal, and in rare cases a mastectomy or hysterectomy as well, sometimes as part of an erotic death slave contract. The end result is literally just a featureless vaginal hole and a urethral opening.
Women who have this done will not experience sexual sensations in the normal way, of course. Depending on the amount of clitoral removal done (generally total), there may be almost no clitoral sensation left. Orgasm is still possible, but usually difficult, although many women find that their nipples and other erogenous zones become more sensitive to compensate.
This type of procedure is often illegal (even for doctors) in a growing number of countries because of blanket laws banning non-consensual FGM.