Originally, they were actual items that were manually applied to each skill. Known Runes included Lethality Rune (boosted Critical Hits), Multistrike Rune (allowed the skill to hit more targets) and Striking rune (effect unknown). Later, each rune was dubbed for its color: Crimson, Indigo, Obsidian, Golden or Alabaster. However, over the course of development, the Skill Trees were simplified, and actual items called Skill Runes were removed. Instead, each skill in Diablo III has a total of five runes, one of which may be selected to augment this skill in a certain way. Most of the time, Skill Runes offer the following effects:
- Change damage (in % of weapon damage)
- Convert instant damage to damage over time or vice versa
- Change Elemental Damage type
- Change the duration
- Add, change, replace or augment the Crowd Control effects
- Remove the cooldown and add a resource cost, or vice versa
- Change resource cost
- Change cooldown
- Change the amount of targets the skill can hit at once
- Change area of effect / range
- Change the attack type (missile to torrent, or torrent to beam, for example)
- Replace damaging effects with defensive ones, or vice versa
- Add a specific stat bonus after using a skill, such as Core stat, Life per Hit or damage done
- Replenish Life or resources after using a skill
- Cause target to take increased damage after being hit by a skill
- Reduce the target's ability to fight back after being hit by a skill
- Change type and / or amount of summoned pets
Once the skill is unlocked, five runes will be unlocked for it one by one, every few levels. A skill rune may be changed at any moment when out of combat and while the skill is not on cooldown, free of charge. However, changing a rune counts as changing skill, therefore canceling all current ongoing effects. Prior to the pre-expansion patch, changing runes triggered a cooldown if not in town, dependent on the difficulty setting.
Some legendary items add a skill rune (or even all possible runes) to a skill, in addition to the one already chosen by player, effectively allowing the skill to have more than one rune at a time. Note that some skill-enhancing effects, and even some runes, are mutually exclusive, as their effects may not be combined due to the way they work (for example, Teleport may not gain both Wormhole and Reversal runes at once; Reversal will be suppressed). It is up to player to decide, and yet some runes may render a specific bonus completely useless. Elemental Damage type for a skill with multiple runes is usually determined by the rune that is selected manually, or by highest bonus to elemental skill damage the player has.
Each rune is visually distinct from others: textures and graphics change with every rune compared to unmodified skills and to other runes of the same skill.
Skills triggered by items may or may not benefit from runes and other enhancements; however, even if they do, they will only do so as long as player has this skill and has augmented it with a rune.