Niko – Master of the Depths
Niko, Master of the Depths is the master associated with delve. He is first encountered in Highgate (Act 4 town), next to the Mines entrance.
When you encounter Niko in the world, the area will contain three sulphite veins. Sulphite is used to Delve into the Azurite mine.
The amount of sulphite mined from each vein is determined by the level of the zone you are in and the quantity bonus of the zone (if you are in a map). Party quantity (the bonus for having more players nearby) will also apply – however, note that the quantity bonus is 50% for each additional player, while the sulphite you receive is split evenly amongst each player. This means that you receive more sulphite in total in a party, but less sulphite individually. For example, with two nearby players you will receive 150% of the initial sulphite in total, which will end up being 75% each.
Sulphite is used to Delve into the Azurite mine. To access the azurite mine the first time, talk to Niko. After unlocking the area, you can get there from any waypoint or by typing ‘/delve’ into chat.
The Azurite Mine was originally designed as a league mechanic, however it’s now been integrated into the base game and is considered by some as an alternate end game to the atlas. The further down you delve the higher level the content becomes and it is an excellent way to make currency (in the form of map returns and fossils).
Once you reach monster level 83 (the equivalent to tier 16 maps) the difficulty continues to increase but monster level does not. The difficulty of the monsters in the mine scales infinitely the further you go down, becoming progressively harder until you inevitably hit a point where it is too difficult to keep going down.
This is where some of the hardest content in the game can be found and it’s well worth your time to get as involved in delve as you can. The rewards are very good for the time invested and it makes for a good break from your general mapping/atlas progression.
The following hidden fossil rooms are valuable
BIOME
Petrified Forest
Sulphur Vents
Abyssal Depths
Magma Fissure
NAME OF ROOM
Stonewood Hollow
Humid Fissure
Crystal Spire
Molten Cavity
CONTAINS
Bloodstained Fossil 
Fractured Fossil 
Hollow Fossil 
Faceted Fossil 
There are some general rules that will help you when trying to find secret walls to break or hidden nodes that require a secret wall.
1 Every square must have something in it. The whole delve chart is separated into a grid, with each actual node holding a single square. This means that along any given path if there is a grid square that has no node it in, it must have a breakable wall. This means longer paths will have more breakable walls for you to find valuable fossils and currency and other items in, as this is the main source of income from delving its important to try to break as many of these walls as possible. This also means that when looking for hidden nodes there must be a minimum of 1 square gap between the node on the main path, and the hidden node as this is for the breakable wall to the secret path.
2 The “1-3-4 rule”. This refers to the fact that any node may be connected to a single other node, 3 other nodes, or 4 other nodes – but never exactly 2 other nodes. If you are trying to find the wall to break to reach a secret node, the first place to look should be at any surrounding node that is connected to only 2 other nodes (and has a clear path through the grid), as these nodes break the rule. This extra connection would make it a 3-way node, which follows the rule. If there is no 2-way node nearby, then proceed to check potential 3-way nodes (which could have an extra connection to make it a 4-way node). In rare instances, two hidden nodes that are close together could come from the same 1-way node, making it actually a 3-way node.
It should be noted that in very rare instances the 1-3-4 rule doesn’t hold due to the layout of the delve (usually as a result of Niko’s mine shaft). So very occasionally a 2-way node can exist, but this should happen less than 1% of the time.
As you delve deeper into the mine, you will find that you need more and more sulphite to sustain your progress. Sulphite map rotations are popular in the community for this purpose, and the following video details how they work.