I found this out in the Hinterlands, a horde raid (which was forming in Seradane to raid Emeriss) attacked me, so i came back and found a way to ruin their raid.

First, Mount up and run to firing range of Emeriss. Fire a shot and run like hell towards the enemy raid, try to get right in the middle of them. Once you're there, let him kill you. (Make sure it's him that kills you, not a player. A mushroom will grow on your corpse and deal damage to all players around you. When I did this it caused a chain reaction which killed about 1/4 of their Raid. Needless to say, you'll get camped for doing this, so have an escape route.