Recommended gear levels for this instance is full T1/T2 dungeon gear with 220 hit/focus or 150 toughness.
River of Souls opened up after the patch 1.1 event finished. The instance is death based which means death resist will help your raid, but it is not a must in order to clear the instance. Many people consider River of Souls tier 1.5 content, but it would be better to consider it similar to Greenscale's Blight difficulty wise. For progression, it would be smarter to rotate progression between the two instances rather than clear Greenscale's Blight first and then River of Souls because it is .5 tier higher. Recommended progression will be listed at the bottom of this page.
River of Souls is an entry level 20 man raid just like Greenscale's Blight. It may have been released later, but difficulty and mechanic wise it is teaching you the ropes of raiding before they begin to make content much harder.
River of Souls item level is a little better than Greenscale's Blight and Gilded Prophecy. You will need to use a mix of all three instances to become "Best in Slot" geared, though. Clearing the whole instance grants you 21 Marks of Ascension which you can use to purchase tier 1 raid gear. The last boss drops relic quality items just like Lord Greenscale, except the majority of them have much better stats/procs on them.
If you complete all of the achievements required for Conqueror: River of Souls you will get a 110% speed mount called Swift Soulbound Charger. The achievements are not very, very difficult, but they are not ridiculously hard either. They serve as a great benchmark for determining whether or not your guild is Hammerknell ready. Alsbeth That Ends Well is not required for Conqueror: River of Souls.
Recommended Progression (in blue are bosses located in RoS):
Duke Letareus --> Infiltrator Johlen --> Oracle Aleria --> Dark Foci --> Warmaster Galenir --> Prince Hylas (for guilds with strong healing)/Plutonus the Immortal (for guilds with strong DPS) --> Herald Gaurath --> Alsbeth the Discordant --> Lord Greenscale