I am not aware of mass cleansing ceremony. The version of cleansing i know..is that those who have killed in war or cattle raid...are cleansed. Not everyone. The ceremony is know as Anyiny (to sweeten)...and as far as i know..during war..it can be done on battle field by elders..so the killer can go back. What normally happen is that once you kill somebody..you're isolated until cleansed...during prolonged battle..they do it on the field or later....at home...where one receives tribal marks for every killed person in addition to cleansing.
During cattle raid like those pokots...when they arrived..those that have killed in battle...are isolated, cleansed (small ceremony) and receives tribal mark for every death..in kalenjin the marks are mostly on the chest...this is i think is nilotic culture.
Cleansing is also done when a kalenjin kills a fellow kalenjin...and cattle compensation dished out.
Kalenjin believe only warriors have not sinned can to go war or raid..otherwise they will be killed. The first vetting is on purity. The sins are many...for instance sleeping with older woman is considered a sin..and anybody who has done so..is told to walk back home..those that have killed previously or in battle...are ineligible..unless cleansed. Otherwise they'll go mad or be killed in battle.
Regarding the regiments..in kipsigis..they are organized first by locality (say administrative division) then regimented into 4 "companies" based on clans. Each company being led by a leader chosen by them. Normally it's one age set waging war or battle..with existing age-set acting as guides and advisors..on the back would be old men...elders..for arrow poison (including treatment) and cleansing..foretelling.
All these are public info...most of it thought during MTC...like is happening right now in my shags.