Runes

Runes are magical symbols carved on rock. They made their first appearance in the Diablo expansion Hellfire as usable items. They were reintroduced in Diablo 2: Lord of Destruction as a new type of socketable item along with jewels. Their role changed again in Diablo III, where they are no longer items, but affect the way skills work.

Diablo I: Hellfire expansion
Runes are trap-like objects that can only be found in the Hellfire expansion. Runes are dropped by chests and monsters and randomly sold by Adria the Witch. They serve as traps, activating a spell. They deal Fire or Lightning Damage, or cast a Stone Curse, on monsters that walk over the activated rune. These runes are not inventory items.

To set a rune, place it in the belt, then click the appropriate key: your cursor will become a pointer. Clicking on the dungeon floor will set the rune, so long as the cursor is within the character's line of sight. The next monster or player to walk over the rune will trigger it, and the appropriate spell will cast from the spot of the rune. Runes can be cast directly on a monster's current location, and will detonate at once.

The damage/duration from rune effects is determined by the player's level. They become more damaging for higher level characters (as is the case with almost everything in Diablo I/Hellfire). The increasing power of runes does not keep pace with the increase in power of other abilities after the early portions of the game.

Diablo II
Runes are a type of socketable item introduced in Diablo 2: Lord of Destruction. The player can insert them into socketed items to add many magical attributes. The type of attribute added depends on the socketed item.

Runes are unique as they can be used to create Rune Words. Rune words are special combinations of runes that bestow not only the benefits of the associated runes but also add many powerful magical attributes due to their combination.

Horadric Cube crafting recipes utilize runes. A and an armor piece combined in the Cube will result in the armor being repaired. can be similarly used to repair weapons. As the repair cost of very high level armor can be many thousands of gold, it can actually be easier to trade for a Ral than collect that much gold. This is additionally useful since repairing is the means by which spell charges are restored to equipment, and such charges can be even more costly than Durability repairs. Other Horadric Cube crafting recipes use runes as an ingredient in making custom items, or upgrading the base item type from Normal to Exceptional, or from Exceptional to Elite (the latter is a Ladder-only recipe).

Runes in the Lord of Destruction expansion were identified by certain mystical words (E.g.: ).

In patched versions of Diablo 2: Lord of Destruction, runes can be upgraded to higher runes by using the Horadric Cube to transmute three same runes much like gems. Runes from to  can only be transmuted with the addition of gems to the recipe; runes from  to  can only be transmuted by ladder characters or in single player. Runes from require only two lower runes (and a gem) instead of three.

Diablo III
Runes in Diablo III provide bonuses to character skills in lieu of items. They have much more descriptive names now and are directly integrated into the Skill screen instead of the inventory.

Diablo IV
There are two types of Runes in Diablo IV, Condition Runes and Effect Runes. Any Condition Rune can be combined with any Effect Rune to create a Rune Word. Rune progression is in the works.

Trivia

 * Although runes and runestones exist in real life, none of the runes in Diablo II resemble any of those used in ancient Norse, Anglo-Saxon or Proto-Germanic alphabets, with the exception of the Jah Rune, which resembles the real-life rune Mannaz.
 * The Diablo IV runes system made of "triggers" and "effects" likely came from the World of Warcraft patch 8.2 feature of Mechagon conditional items, where an item with a cause could be combined with any item with effect to produce a customized semi-passive bonus.