Velious Leveling Guide - Siren's Grotto
In the water tunnels you'll find a large amount of fish which are good for players 55 - 60. The swordfish see invis and you'll also have a chance of spawning Fellspine a named who drops Fellspine's Belt.
On the land you'll common find seals and ulthork enemies which are also good for players 55 - 60.
Inside the fortresses on land you'll find sirens which are a bit tougher than the other enemies so I recommend you be 57 - 60 at least and well prepared.
The hardest part of the zone and most rewarding would be where it says Mistress Latazura on the map above as well as the cluster $%^& up top on the map. There's an underwater tunnel you can reach in the northern part of the zone by the Western Wastes ZL, it's ontop the land mass north of the water fall and it goes straight down. Inside here you'll find a whole city of sirens.
Additional Zone Information