(rough guide based on Alpha info)

The River of Souls event starts today March 30th, and will run through no later than April 9th.

There will be a special 'currency' that drops from events that are part of the River event, this 'coin' can be used to purchase special event only related items as well as items that will no longer be available for purchase after the event.

Step 1: Each of the 11 zones will have 3 Bosses that spawn as events. Contribution in these will earn the above 'coin'. Once all 33 of these new bosses are defeated the next phase will unlock. (note: even after the River ends, these bosses will be added to the zones events list permanently)
These bosses are said to be "roughly twice as hard as the normal bosses for their zone".

Step 2: Once the 33 Bosses are downed, 5 more bosses will spawn in Stillmoor. These 5 Bosses are described as 5 man expert bosses on crack. BUT they are open world and can be gang tackled and raided.

Step 3: Once the final 5 bosses are dead the portal in to the "River of Souls" Raid instance will open, and remain open from then on. All those in the instance at the first kill of Alspeth will receive special items, including the Epic version of the Spectral Horse.

After the completion of the event those items specific to the event will only be present as rare drops from the new zone bosses.