Sealed Corridors (2,849,276,187)

One of the Perandus challenges requires killing Vaal bosses, I've killed all but Wiraq the Impaler in Sealed Corridors.

I've been grinding through Scepter of God and Upper Scepter, 2nd floors respectively, on both normal and cruel on different characters for a week now trying to find it without luck while the other areas I found in an hour or less. Is Sealed Corridors bugged or is it unbelievably bad luck?

Just luck. You can try running sacrifice pieces in the map device but that's random which one will spawn too.

Try merciless? Dunno if the spawn rates are different per difficulty, but I usually find one corrupted zone per session when I'm either farming domi or looking it for challenge like you were.