First:
I can pick up DS almost every time I come across it
'"Bah!" at you ;)
That being said: I'm not sure, since I've never accomplished it, but I seem to recall this is key...
http://doom.chaosforge.org/wiki/Azrael%27s_Scythe
I could say more than just that, but this should enable you to shake hands with... it.
I've never wielded Dragonslayer or worn Berserker Armour or encountered an Apostle, so I can't speak from experience, but rumour has it....
If you're wielding DS and wearing BA when you get to the last level in Ao100/ArchAo666, an Apostle appears. And in the regular game, he takes the place of JC if you're using DS/BA when you get there.