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 a version of the Labyrinth, the player will receive two Ascendancy skill points to allocate in the ascendancy skill tree. The first three Labyrinths can be unlocked via playing through the Acts, resulting in six Ascendancy skill points. Two more skill points are unlocked when the Eternal Labyrinth, which is unlocked post-game via maps, 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 - aside from the Ascendant's - also follow a certain format. Most classes have 12 passive skills in each tree (with some having 14), 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 Eternal 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 (plus two skill points) requires 5 points to reach, which is gated by a free skill point. After clearing the Merciless Labyrinth, the Ascendant can have either 2 Ascendancy Class passives or 1 Ascendancy Class passive and its base class's starting point. After clearing the Eternal Labyrinth, the Ascendant can have two Ascendancy Class skills plus one class's starting point, which results in 5 free skill points. The Ascendant can choose not to take a second starting point and take an attribute skill and a skill point instead, but this is not recommended as two free skill points is almost always worth more than attributes. Consequently, the Ascendant does not reach a power spike until completing the Cruel Labyrinth and cannot start allocating points from another class's starting point until completing the Merciless Labyrinth.
Refunding one Ascendancy skill point requires five regular refund points. When all points are refunded, use the Altar of Ascendancy in the Labyrinth treasure room in any difficulty to select a new Ascendancy class.
The Ascendancy classes are introduced. If you came back to the game with an older character then you can obtain the Ascendancy Skill Points in the following ways.