Steam rune

Steam runes are runes made by combining Water and Fire runes. This can be achieved by taking
 * pure essence, Water runes and a Fire talisman to the Fire altar or
 * pure essence, Fire runes and a Fire talisman to the Water altar.

These runes count as one of each type used to make it. Combinations have a 50% success rate and the talisman is consumed in the process. This is one of the combination runes released with the abyss update.

Wearing a Necklace of Binding increases this success rate to 100%.

Reference
Knowledge Base on Crafting Runes