actually herald is a FoS now (feat of strength) so i dont think you'd be able to find it in the achieve window....but it is on wowhead.
basically you have to go to ulduar and kill algalon with only 80s in the raid, on 10 man, and using gear no higher than ilvl 226 (except weapons, those can be ilvl 232)
if you can do all that, where the "hardest" part should just getting the drops you want, you get rewarded with your FoS and the title "%n, Herald of the Titans"