The Tomb of the Forgotten King is a dungeon located nearby Owl's Reach. the dungeon is locked, and only those with its key can enter, as a Protection flag makes any other way impossible. The dungeon contains a vault known as the Vault of Emerillion. This is in fact the first dungeon Runt and Breeze cleared (wv14-15/8bw5).
It's a mix of puzzle and combat, requiring one to use both their weapons and wits in order to pass. The first room has the adventurer kill various mobs (mostly zombies) and drop the loot down a nearby shaft to activate the door and pass through.
The second puzzle is similar, instead of having the adventurer drop loot down to a nearby shaft, it tests their archery skill by having them shoot another pressure plate. The third room is a lethal guessing game, with a long hall filled with pressure plates. One misstep, and the ceiling comes crashing down, crushing anything over one meter in height.
After those and a challenge of passing through a series of cobwebs, the adventurer will stumble into a series of vast chambers, where they'll fight mobs all the way up to the boss room, and into the waiting arms of Nethersoul, a massive zombie with high melee damage.
Some of the boss loot, as seen in Wimpy Villager 15/Diary of an 8-Bit Warrior 5, consists of a Golden Cow with Prosperity III, a Bed of Roses with Goodnight I, an Enderpouch with Pocket III, and a Dragonskull Helm with Resilience VI. According to S, the items generated are random, so the items above are just examples of boss loot.
Nether soul also drops 1 quest items it being vital in the current war in Aetheria, The vital item is the Sapphire Flame, which is used to light a beacon at a temple at the top of the Spines of AO, the tallest mountain range in Ardenvell, although it a very difficult challenge. Once the beacon is lit, all the villages around the continent will know to start preparing for war.