Access the boss

To access this boss, you have two options. The first is to go through the Dark Deepholm area, be careful you will take damage by jumping to reach it. The other is to go all the way around, going through the jump zone rather than the acid zone to avoid taking unnecessary damage.

You then come to an area with petrified drow and an altar. Be careful when you arrive at the altar, the fight will start with the appearance of the Spectator boss. You can kill the petrified drow with spells beforehand, they will be enemies in the end anyway (unfortunately no way to help them!).

Do not try to remove the charm, the charm means that they cannot attack the caster. I also tried to charm them, but that does not make it possible to rally them.

Kill him easily

The fight can be quite complicated, if you are not well placed. Separate your troops to start the fight with a single character. Aim preferably at Commander Maz'ri (she hits hard and has little HP), then the Boss Spectator. Finish with the remaining drow.

Avoid showing yourself to the boss for the duration of the fight, he will then favor his attacks on the drow.

Booty and epilogue

You will have plenty of loot, but unfortunately of poor quality. You will also have a crumpled missive but currently no quest to follow.