The Undercity Ruins are reachable from the Lower City Sewers and can be found on the very northwest part of the map.
Once you make your way to the Undercity Ruins waypoint that is still part of the Lower City Sewers, you will find a door that cannot be lockpicked.
Opening this door can be done by shooting one of the hanging corpses as you stand below them after a puddle of blood is created.
Surviving The Undercity Ruins Ambush
The ambush begins once you head down the steps after you go past the body of a dead human named Justen, who is most likely another victim of an ambush.
You will be attacked by 3 Reapers of Bhaal from a distance while also being targeted by Farslayer of Bhaal Ghislev. (Farslayer Ghislev is the real problem here)
To avoid getting surprised by their attack, you can cast ranged spells at the location where they are hiding, and this will give you the upper hand.
The area where the ambush takes place will be marked and any party members within the area will be affected by the Bhaalist Ritual Target condition.
This provides you with a limited number of turns before Ghislev kills certain members of your party who have 100 or fewer hit points. (This will happen instantly once the condition ends)
Leaving the marked area will allow you to reset this and you can also escape from the fight with Flee Combat and reenter the area by sneaking to get by unnoticed.
Once you manage to make your way past the first 3 Reapers, heading down the stairs will result in another ambush and your enemies will have the high ground.
Ranged attacks work well and you can also move to where they are by jumping or casting spells that move your character to one location.
During this part of the ambush, you will encounter Strangler Luke who has the Monk class and a habit of knocking his targets into the chasm. (Avoid the bridge until the fight is over).
Crossing the bridge after the second ambush may seem like the next step but this would be a bad idea as there are more reapers ready to fire arrows at you from the windows.
Instead, you open the door where you fought Strangler Luke as this leads to a path that will take you to the reapers by the windows, giving you the chance to surprise them this time.
Once you have fought across the bridge or have snuck through the other side to kill the reapers by the windows, you will now need to head for Ghislev.
When any of your characters get closer to Ghislev, he will teleport away to a nearby Metal Door to bait you.
This is where you will have a fight with Manglin’ Abby and she also has a habit of knocking your characters into chasms.
Keep away from the chasms to avoid instantly getting killed and take out Manglin’ Abbey to leave Ghislev helpless.
Killing Ghislev will result in all the enemies that are alive or that have been killed disappearing, which prevents you from getting loot. (It is best to loot as soon as you kill them).
Verdict
Avoiding most of the fighting is not possible but you can engage enemies in a manner that will give you an advantage, which often involves striking first.
Should you have characters that have more than 100 hit points, they won’t suffer from the effects of Ghislev’s ritual and may easily fight their way through the ambush.
Keeping your other party members out of the ritual area may be a good way to keep them alive but this will handicap you by reducing the number of characters you have in a fight.