Mist rune

Mist runes are combination runes. They count as two separate runes: one Water rune and one Air rune. Thus, any spell requiring one Water rune, one Air rune or both will spend only one Mist Rune.

Runecrafting Mist Runes
At Runecrafting level 6 or higher, they can be crafted in either of two ways:
 * Pure essence, Water runes and a Water Talisman to the Air altar
 * Pure essence, Air runes and an Air Talisman to the Water altar.

Players receive 8.0 experience for combining water runes at an air altar and 8.5 for combining air runes at a water altar. Combinations have a 50% success rate (or 100%, if the player is wearing a binding necklace), and the talisman is consumed in the process (unless the player uses the Lunar spell Magic imbue).

Miscellaneous
This is one of the combination runes released with the abyss update.

Mist Runes are used in the quest Swan Song.

The Mist Rune is perhaps the most reliable type of rune for using a combination attack due to the spell cost for attack requiring an equal amount of runes for each use of water spells.//SEE DISCUSSION ABOUT THIS HIDDEN TEXT

Spells

 * Water Strike
 * Water Bolt
 * Water Blast
 * Water Wave
 * Water Surge
 * Falador Teleport
 * Mobilising Armies Teleport

Trivia

 * As with all combination runes, the most amount needed for a spell is 30, for the Charge Orb spell series.
 * The Mist runes inventory icon, along with the other combination runes, was unaffected by the update on 9 June 2010. The other runes were slightly twisted.

Reference
Knowledge Base on Crafting Runes