So after reading A Fistful of Warlocks I believe that I have found out what Cowl's necromantic abilities are as aside from the Darkhallow he has not used any so far in the series. First all of the apprentices seem to have a similar skill set with evocation as Grevane and Corpsetaker have both used lightning and kinetic/force energy in the stories they have appeared in with Cowl having the most gift in force magic. In the thaumaturgy side, Grevane used the more traditional necromancy, raising zombies as his forte while Corpsestaker used ghosts and spirits, both abilities that Kemmler himself was a master of. The last ability that he was most well known for was rising from the dead multiple times. Previously Kumori told Harry that Cowl has contended with death curses before and as he seemed the most powerful and more importantly intelligent of the three, he most likely adapted Kemmler's most useful ability while maintaining a disdain for the other sides of necromancy. Knowledge is power and this also gives credence to the Simon is Cowl theory as he would have no need to fake a death curse if he actually died and he would be able to be more active in the Black Council as events in the series have escalated to say the least.