Trapped! My Story.
I found if you don't have attacks/spells/abilities that are also powerful that have a 1 target, 1 radius or 1 direction it can be hard to 1. kill the assassin lord and 2. keep the merchant alive in the same playthough. However I hadn't killed the other enemies first before aggroing the assassin lord so not only was the merchant very close but all the other bandits turned hostile at the same time and were attacking me along with the assassin lord.
Also, if you have Retaliation damage and the merchant has been aggroed it can be impossible to do those two things because the only thing the merchant has going for him is that:
- He doesn't usually get directly attacked by the enemies (even if he is aggroed).
If you, the player have Retaliation or pets it is normally enough to finish an aggroed merchant. An "aggroed" merchant was friendly but has become hostile to your side because of the damage he has taken - he has a red border and right-clicking will show his reaction which in this case is hostile.
(GENERAL TIP: Right-clicking on any creature including your player will give you more information. It is good to do often, especially as you gain more knowledge of different abilities and effects by playing more classes and taking part in the community.)
Either remove any items/effects that grant Retaliation or be very careful not to let any damage happen to the merchant. Pets/servants/summoned creatures will not attack the merchant if he is friendly, however will quickly finish him off if he becomes hostile to you - including if they "accidentally" damage him or he is already hostile and he attacks creatures on your side which have Retaliation. Don't rely on pets not attacking the aggroed merchant - best not to turn the merchant hostile in the first place.

In my Higher (Higher is a Subrace of Race: Human) Writhing One (note: Writhing One is a Forbidden Cults DLC only Class) most of my talents and abilities were for 2 or 3 radius and I only aggroed the assassin lord by talking to him - which meant the merchant was close and would have got killed easily if I had used them. Also I hadn't killed off the other enemies beforehand.
In the end I managed to kill the assassin lord after losing one life by confusing him with one of my class abilities and finishing him off. Then I killed off the other enemies and went to one of the chests and opened it - bad idea (TM).


After I foolishly opened the chest and a large number of enemies popped out, I couldn't exit the exit (which appears on the tile the Assassin Lord dies on) because of negative statuses. Luckily I was able to survive until I could successfully exit with the Lost Merchant still alive.


Normally chests won't spawn more than one monster (even though one monster can be enough to kill you

Main ToME/TE4 Forum Links:
[Chest and Vault Tips.]
[Spoilers: Age of Ascendancy Campaign (AoA) One-off Locations.]