The drops all appear to be 160 stat armor pieces. This zone has gorgeous artwork.
Siren's Grotto: Tavalan Abyss has an intrinsic debuff that lowers your crit chance by 100%. In addition, monsters found within the zone have an intrinsic buff that raises their crit avoidance by 100%.
All the Tavalan squidmen found in the zone also have a nasty trauma debuff. Either have a quick finger on the cure button, a big stock of trauma cure potions, or both. If you see them start to cast it is advised to back up as it seems this debuff that will kill you quickly has a short cast range.
Make your way up the entrance tunnel. The coerced Combine soldiers can be ignored, it's safer to kill the Tavalans. The tunnel lets out in a big central room called the Hall of Summoning. There's an odd sculpture in the middle of the room surrounded by Tavalans. Kill them.
From the Hall of Summoning, two paths lead west, but the upper one is gated. There's an elevator due west, but it's sealed. Head down the lower path, clearing Tavalans. Again, the cocerced Combine soldiers can be ignored unless you want the experience points.
The path goes through the Temple Stones. Mindscorcher Shellik is on a raised platform to the south. Shellik summons soulscorchers. He will periodically emote "You are no match for my brilliance". About 5 seconds later he'll cast Mindmelt for automatically lethal damage. Joust to avoid. The trick to this fight is either having enough DPS to burn down Mindscorcher, or enough AOE DPS to handle the soulscorchers.
Shellik can be temporarily bypassed.
At the end of the path, Baritum, Cartis, Deklani and Farinu attack as a group. Their classes are random. Pay close attention to the intro text as this will tell you which is which.
The Healer: "Channeling the power of fear into my brothers!" Unless you kill the healer first, he will heal any of the others once they get below 80% health. He can not heal himself.
Once the healer is down, it doesn't matter much what order you take them out in.
There is another guy that will force you to not target anyone and will not allow you to target for a few seconds. I believe this ones says, "I am the bringer of your doom. Feel our wrath!"
At some point she will unleash two adds - Baraki the Abominable and Otyug the Noxious - both normally inflict crushing damage. Otyug can cast Eerie Gaze and Phosphorous Spittle. Kira has a Controlled Repulsion buff (?) that inflicts damage on the group. Kill the adds, then Kira.
Use the conch you got from Kira the Temptress to open the 3 portals back in the Hall of Summoning. Note: If you have left the instance, the conch goes away. If you re-enter the same instance, go back to Kira's room and burn down "Kira's Spirit" (easy kill) to get another conch. Engage her and 10 seconds in you see red text. Click on all three seashells to open them. This turns on the portals to prevent her epic minions from coming in to kill you. Burn her down to about 10% and you will see more red text that a portal is opening. One of the 3 shells will not have the blue beams coming out. Click that shell to re-enable it. Burn her down the last 10%.
Kill the mischievous frightlings or they will add to the fight. The Overlord is ^^. Periodically during the fight he will create a duplicate image of himself that is ^. The original Overlord is rooted while the duplicate is up. Pull the duplicate away and destroy it to prevent it from healing the Overlord.