When you wear DS/BA and enter floor 666, then every monster in that floor is instantly killed and some odd message appears.
"You feel something is wrong.. really wrong!"
and Apostle comes out.
(I think you can also meet him at the 100th floor of Angel of 100, or nukes SMM in standard game, provided that you are wearing DS/BA)
It is a kind of super Archvile, and hits hard. Its HP is not very much.
(Cyberdemon x 1.5~1.6, I guess, since DS kills Cyberdemon in 2~3 hits, Apostle got 4 hits and died)
ending screen