Skill trees complement RPG narratives, empowering the game player to upgrade their avatar. One of the most important factors when considering skill tree design is the degree of player agency within the game. For example, Spider-Man (2018) implements a limited skill tree, allowing the player to develop a specific fighting style, while World of Warcraft implements a more flexible skill tree, because players are not confined to a single titular archetype (i.e. Spider-Man); rather, they can elect from a variety of player races/factions.

Contents
1. General Skill Tree Anatomy
2. Diablo Skill Tree
2.1 Skill Types
2.2 Skill Organization
3. World of Warcraft Skill Tree
4. Path of Exile Skill Tree
5. Takeaways
6. Sources

General Skill Tree Anatomy

• Top tree nodes are the easy-to-acquire skills from which a player develops more advanced skills
• Each of these nodes defines a skill category, such as player health, magic spells, combat moves, etc.
• Gating mechanisms define when a player can acquire a skill; good game design ensures skill acquisition progresses in a balanced way (@VannStrasen is researching how to balance skill trees in Sprint 1)
• Skill tree nodes should communicate the effect of the specific skill, so that the player knows how to use it; some skill tree designs even contain previews of skills to follow in each existing node, so that the player knows what to expect/look for
• It is best if the skills acquired do more than give power to the player, by actually changing the way the game is played (e.g. acquisition of combat skills renders the game more dangerous by also introducing more enemy characters?)
• Consider the extent to which skill trees should be allowed to be reset (fewer resets means more re-playability, as the gameplay will vary more, but can frustrate a game player if they make the wrong choices for their desired skill tree development)

Diablo

• Total number of skills a player can possess at any point in time is limited (a design choice that enhances/complicates gameplay)

Skill Types

• Skills are categorized into Active, Passive, and Active/Passive
• Active skills must be manually activated and can either be Channeled (after activation, the player must maintain the effect of the skill, a spell cast, with a repeated action) or a Summon (a minion character will materialize to help the game player in their battles/endeavors)
• Passive skills do not need to be activated; once acquired, their effect is lasting or triggered by other events in the game
• Active/Passive skills exhibit a blend of active and passive skill behavior (e.g. "auras" have a passive effect but can occasionally be activated for player bonuses)

Skill Organization

• Diablo skill trees are navigated with tabs that categorize the trees into "Druid Skills," "Elemental," "Shape Shifting," and "Summoning"
• Each tree displays the skills in rows, with each row signifying the level at which they become available for acquisition (the top row of skills is available at the game avatar's inception)
• Arrows between skills indicate which skills are prerequisites for others
• Possessing certain combinations of skills can passively enhance the effect of those skills

World of Warcraft

• Skills for professions can be "created" or "harvested," while riding skills must be purchased from riding trainers (i.e. active skill acquisition mechanisms, via construction, training, quest completion, or purchase, as well as chance skill acquisition mechanisms)
• The ability to use specific tools (e.g. weapons, armor) depend on the skills acquired by the player; skills indirectly empower the player by providing access to advanced technology
• Player races determine the starting skillset and the class of skills the player should strive to acquire (e.g. as a death knight, rune-forging skills are a prerequisite for engraving runes on weapons)

Path of Exile

Methods for Acquiring "Skill Gems"

• Earned through a quest reward
• Purchased from town vendors once a certain quest is complete
• Vendor recipes
• Random drop from monsters, chests and strongboxes
• Vaal skill gems are obtained from Vaal Vessels in corrupted areas or by corrupting a regular skill gem
• As a reward for turning certain sets of divination cards

• Skill gems also have associated levels, so even after a skill is acquired, the skill can be upgraded to higher levels to result in better performance (usually by collecting experience points associated with using the skill)

Takeaways

Ultimately, the features of a good skill tree include:

• A unique skill tree for each distinct category of skills, unless the game supports only one category of skills (e.g. only magic spells)
• Gating mechanisms that determine how/when a skill can be acquired
• Short comments on each skill node with a description of the skill (what the player can use it for) and perhaps a hint at what the player can do to acquire the next skill after it
• Limited skill tree resetting abilities: a Chiventure game developer should be able to determine whether a player of their game can hold unlimited skills or only a limited number of skills at a time. If only a limited number at a time, the option to possibly remove skills to make room for other ones should be available. Otherwise, the game player will always have to face the consequences of electing certain skills over others, with no room for skill-selection error.

Considerations for Design/Implementation of Skill Trees in Chiventure

• We should have templates for how Chiventure game developers can organize their skill trees, whether with only one category of skills or several, as well as templates for how skill trees should be displayed on the command line
• Chiventure game developers should be able to specify the foundational skills a player of their game begins with, and, depending on the progress of x/rpg-playerclass, we could also support defining different starting skills for each player class.
• We should have gating mechanism events. The analogy here is to JavaScript events like onClick or onLoad, etc. So, in Chiventure, we could have onPurchase or onQuestCompletion as events that trigger skill acquisition. We would have to work with x/action-management, x/custom-scripts, and/or x/wdl to decide on how these events would be specified. Also, currently, there is no x/rpg-quest team, so onQuestCompletion is just an example; its implementation will have to be considered by future generations of Chiventure developers.
• The skill trees library of functions should be focused on triggering skill acquisition (see above point) and supporting skill usage by game players.
• Down the line in Chiventure development, with multiplayer support, we should also be able to support the ability of players to trade skills.

Sources

Skill Tree Overview

A good summary of skill trees in well-known RPGs

A good summary of good skill tree design

Specific Skill Trees

Diablo Skill Tree

World of Warcraft Skill Tree

Path of Exile Skill Tree