So I helped get this drop for two necromancer's this weekend. It took 5 kills of Keeper of Souls to get two capes.
We were actually lucky enough that it dropped the first time for one, but then took 4 more kills to get it again. lol
Best way to do it, only kill one Pegasus or griffon if possible, then camp out, wait about 60-80 minutes for KoS to spawn. Come back in, kill KoS. If no cape; kill another Pegasus or griffon. Camp out, about 60-80 minutes. come back KoS should be up again, kill. Rinse, repeat, until you get the cape or whatever drop you're after for that matter. I spawned him 3 times just this morning doing this.