This is a very hard mission, its not for the faint of heart and you absolutely need to play it. You may need a guide, or dive deep enough to read a LOT of readables, but in the end the experience is worth it.

There is a lot of love in the mission. The ambiance is one of the best of any FM out there, author knows how to be subtle. The design of rooms is sublime, everything is thought out and exploration always yield rewards. When you think there will be nothing in a forgotten corner of a crate, you are always surprised with a reward that is lore and location appropriate.

Downsides, aside of complexity in my opinion is that the author tried to do too much at once, we have a glorious "Haunted Warehouse" theme, akin to 13 ghosts where something went wrong and the warehouse went into lockdown...But...
- There is also a mineshaft under the warehouse
- There is also a precursor temple under the warehouse
- There are also burrick tunnels
- There is a huge safe heist
- There is a runaround with crafting of an item
- There is a condescending demon that teleports you around the place
It works when its organically fit into the overall location like an outburst of wild growth in one of the parts of the warehouse, but it stretches the narrative in other biomes. to a point where it becomes a bit exhausting.

Some objectives of the mission only make sense when you read into everything, I stumbled upon combining two cups to get the cup of balance, even though the entire level was hinting towards it with a giant balance sign on the floor. The level design pushes you to blow up some mine wall bits with "X" markings in the tunnels using explosive barrels, but after getting nothing from the first experimenting attempt, you give up, only to find out from the walkthrough that there are many such places and they yield a crafting component you need to open the vault. Its a bit Zontik-esque, but it can be forgiven.

Lingering Whispers is also a perfect category of missions that avoid the arrested development trope. As the mission progresses, new enemies enter the fray, security system come online that increase the overall challenge level. All wonderfully foreshadowed and explained lore-wise.

The brushwork has some of the more innovative things compared to other missions, attention to detail, surfaces, see-through zones are lovely. If you think you can go somewhere, you can, and there is reward waiting (or a teleporting demon face).

ScaryKitties did a great developer walkthrough and a guide you can check out of the ttlg forums. The author didnt release new missions for almost 12 and a half years, but this work shows a sign of a pure legend one step away from the pantheon of best mission makers in the community.