Nineteen souls survived the Trials of Ascendancy. Nineteen men and women stood at the gates of the Lord's Labyrinth. Nineteen challengers whose exceptional martial and mystical talents still linger long after their death.Description on the official Path of Exile website 
Choosing an Ascendancy class after completing the Labyrinth for the first time allows the player to gain access to a new Ascendancy skill tree specific for the class chosen. Each character class comes with three associated Ascendancy classes (except for Scion, who only has one) for a total of 19 Ascendancy classes.
The first time the player completes the Labyrinth on each difficulty, the player will receive two Ascendancy skill points to allocate in the ascendancy skill tree. Two more skill points are unlocked when the endgame Labyrinth is completed, which means players can spend a total of of eight Ascendancy skill points.
Ascendancy skill tree
The Ascendancy skill tree is a small skill tree that a character has access to when the Labyrinth has been completed. Each Ascendancy class has its own tree. It functions similarly to the larger passive skill tree; the player earns Ascendancy skill points, which can be spent on allocating nodes granting passive bonuses. The Ascendancy skill tree is shown on the regular passive skill tree, in a tab near the character's starting location.
Ascendancy passive skills are significantly more powerful than regular passive skills, and will heavily impact the way a character is played. A character can only spend a maximum of eight points on the tree; however, the last two points are not available until significantly later when end-game maps can be cleared. The skill trees - disregarding the Ascendant's - also follow a certain format. There are 12 passive skills in each tree, and each potent "notable" skill is gated by generic, lesser node, so it takes two points to reach them. Certain notables require the player to allocate another notable first, so they take up four points. A character will only be able to have one four-pointer skill until the endgame Labyrinth is cleared.
The Ascendant follows a different format: each Ascendancy Class passive takes 3 point to reach, gated by an attribute passive then a free skill point, and obtaining a second starting point requires 5 points to reach, which is gated by a free skill point. As a result, the Scion can either have up to 2 Ascendancy Class passives or 1 Ascendancy Class passive and its base class's starting point plus an additional attribute passive. Consequently, the Ascendant does not reach a power spike until Cruel and cannot start allocating points from another class's starting point until Merciless. After the endgame Labyrinth is cleared, the Ascendant can either have two Ascendancy skills plus one class's starting point or two Ascendancy skills, a skill point, and an attribute passive. The Ascendant can gain up to 3 free skill points after Merciless Labyrinth if they choose not to take an main ascendancy notable. This is usually not advised as the Notable passives are typically stronger than any one individual skill point in the main tree. Later the Ascendant can have a maximum of 4 free skill points after the End-game Labyrinth if they do not wish to take a new starting point. This can be advantageous for Ascendant set-ups who do not need the new starting point on the passive skill tree.
Refunding one Ascendancy skill point requires five regular refund points. When all points are refunded, you can use the Altar of Ascendancy at the end of the Labyrinth to change your Ascendancy class for free.
The Ascendancy classes were introduced to the game with Version 2.2.0. If you came back to the game with an older character then you can obtain the Ascendancy Skill Points in the following ways.
- Complete the Labyrinth on Normal difficulty to receive 2 Ascendancy skill points.
- Complete the Labyrinth on Cruel difficulty to receive 4 Ascendancy skill points at once.
- Complete the Labyrinth on Merciless difficulty to receive 6 Ascendancy skill points at once.