The Labyrinth (also referred to as lab) is amongst the most important places progression-vise in Path of Exile. It is mainly connected to the ascendency tree, which we covered in another article. 

So, what exactly is the Labyrinth?
To keep it simple, it is a dungeon-esque map full of dangerous foes and traps and a general "we want to kill you" vibe. Its layout changes every day so it has an endless variety to offer. When you finish a lab, depending on its difficulty, you can enchant a piece of your equipment and "ascend" with your character. Also, there are special chests opened by keys you get on various conditions. The boss of the labyrinth, Izzaro deals tons of damage and can wield three different weapon combos (changing daily as well) and is just overall a menacing foe. During the labyrinth you will have to face him three separate times. The lab is a one-shot adventure, meaning if you die you got to start from the beginning, however, you can attempt it as many times as you like provided you are in a softcore league.

At the end of the labyrinth, after you defeated Izzaro, you can finally enter the Ascendency Chamber. Here you will find 10 chests of Izarro's treasure (requiring keys you can find during your lab run), the Altar of Ascendency, and a Divine Font. The former is used to select your ascendency class and the latter for enhancing your items.

You need to make four runs in the lab to fully unlock your character's potential in the ascendency tree. The first completion on normal difficulty will let you choose your Ascendancy Class with two Ascendency points ready to be used. Each of the seven characters have different options to Ascend to. Further completions on increasing difficulties will provide additional two Ascendancy points up to a maximum of eight points.

Now that you know what the labyrinth is you may wonder, Okay the lab is cool, but how do I get to it?

Worry not friend, because accessing the labyrinth is quite easy. As you journey through Wreaclast you will encounter Trials of Ascendencies containing certain traps. The fun part comes from the fact that there are four difficulties for the lab, these being normal, cruel, merciless and eternal. To unlock further difficulties, you not only need to complete their associated trials but the previous difficulty as well. 

The labyrinth provides plenty of worthwhile activities besides the ones already mentioned. There are gauntlets and puzzles, which work similarly in a way. Both of them have a nice chest for you if you complete them and both will try to kill you in selected ways. This may take form in a short and dangerous trap gauntlet or push gauntlets where you need to escort a totem while monsters are swarming you. In regards to puzzles, they offer you classics, like switch puzzles or floor puzzles. Another mechanic to keep an eye for are darks hrines, which give you various boosts for the full length of the labyrinth once acquired, so make sure to pick them up when you have a chance.
If you ever wanted to be known as the best at something and for people to admire your unmatched skill I think you will like the fact that the labyrinth has leaderboards too. To be eligible for a leaderboard, a player must enter the labyrinth alone. The labyrinth must be completed before the daily reset to the next labyrinth layout. There are leaderboards for all four labyrinth difficulties in each league.
Players' completion times are measured from when they start the Labyrinth at the pedestal until they kill Izaro.
To be eligible for the normal leaderboards, players must be level 40 or lower.
To be eligible for the Cruel leaderboards, players must be level 60 or lower.
There are no level restrictions for the Merciless and Eternal leaderboards.
At midnight UTC, the following prizes will be awarded to the players who finished the Labyrinth the quickest that day:
Emperor's Cunning will be rewarded to the top player in normal Labyrinth
Emperor's Wit will be rewarded to the top player in Cruel Labyrinth
Emperor's Might will be rewarded to the top player in Merciless Labyrinth
Emperor's Mastery will be rewarded to the top player in the Eternal Labyrinth