Go to the Foyer bonfire and take the elevator on the left going up, follow it all the way up and get off on the right. As soon as you get off, you'll encounter a second elevator going up, get on that and go left at the top (if you're facing the wall it's left, if you're facing outward it's right) and open the gate leading to another elevator (you need the Tower Key). Take this elevator up and you'll find the group.
Four more will spawn when you try to go up the stairs. There is an Ashen Idol nearby which heals enemies in a similar fashion to Warmth.
Several of him will spawn at once, and he will not count as being vanquished until all of them are defeated. Any that are killed will not respawn. The first one seems to have more health than the rest, but doesn't use a bow. Don't try to take on the other 4 at once, as they will pummel you with arrows or use their katanas which cause bleed. Instead, draw them out with a throwing knife or bow 1-2 at a time and take them out that way.
If you get overwhelmed, the elevator is a quick escape as they won't follow you, or will fall to their death. This only works if it's already on its way up, however.
The encounter is designed to take advantage of the player's natural instinctive reaction, which is to retreat back to the elevator when they see the additional Prowlers spawning, thereby putting themselves into a very disadvantageous situation with very little room to maneuver. If you instead run past them as they're spawning, you can light the Smelter Throne bonfire before they reach you and make the fight much easier for yourself by kiting them around the large circular room. And if you do die, you'll respawn at the bonfire you just lit.
- All of them can drop pieces of the Mad Warrior Set or the Berserker Blade. They do not share the Mad Warrior's special loot table, however. They can all drop duplicate items (eg five Mad Warrior Masks from five Prowlers) and anything they drop will not affect what Mad Warrior drops.
- Equipping items that increase item discovery chance to get more drops from them can potentially allow forgoing farming the Mad Warrior at Belfry Sol, but it's also possible you just end up with duplicates.
- Because of the format of these invaders, they will not drop any Awestones.
- Using a Bonfire Ascetic on the Foyer bonfire in Brume Tower will respawn them.
- Does the Hurrah! gesture upon killing the player.