The Cumdumpster class is obtained by having semen ejaculated into an orifice which already contains semen from a different source.
headgear, the 'bitch bottoms' and 'bitch top'
The quest requires you to have semen from several different sources inside your face, womb and asshole.
The headgear has a special magical effect which makes you suffer no negative side effects from holding liquids inside your butthole, but also makes the expulsion of stuff from your butthole frequent and automatic, in small spurts, and with you rendered unable to expel it manually. Wearing a butt plug will prevent automatic expulsion. You gain endurance (lower fatigue gain and rapid resting) from having cum in your stomach. You gain increased magic damage from having cum in your pussy and asshole. You get sore less easily.