reapers sweep info please

do i need to cap it? i unlocked it wit factions but i couldnt buy it from a skill trainer please help

Yep, you need to cap elites even if you've unlocked them with faction.

Here's the boss that has Reaper's Sweep, aswell as a map to him..
http://gw.gamewikis.org/wiki/Dabineh_Deathbringer

or you could buy a elite dervish tome to unlock it on that char

You've got like 5 active threads all about dervishes (and most are you just repeating yourself). Did you know there is an entire forum dedicated to Dervishes?

Campfire > Dervish