Rune of Holding

A Rune of Holding will, when used on a bag, increase the holding capacity of that bag from 5 slots to 10 slots.

Note: Runes of Holding will not work on Belt Pouches.

Acquisition
Runes of Holding can be bought from all merchants in all towns in Post-Searing (except for Ascalon City and Lion's Arch) at a fixed price of 500 gold.

Tip: After you leave Pre-Searing and arrive in Ascalon City, the easiest and safest way to max your inventory space (assuming you already have got a Belt Pouch) is to buy 2 bags from Arnaud, the merchant in Ascalon City, then walk straight over to the Great Northern Wall (not crossing any hostile territory) and buy 2 Runes of Holding from Rolf, the merchant at that location.

In Cantha, a number of quests give special rewards that may be traded for a Rune of Holding. The collectors can be found as follows:
 * Monastery Quartermaster (Shing Jea Monastery)
 * Imperial Quartermaster (Kaineng Center)
 * Kurzick Quartermaster (House zu Heltzer)
 * Luxon Quartermaster (Cavalon)

In Cantha, you can also purchase a Rune of Holding from merchants in any town after Seitung Harbor.

Tips for non-prophecies characters:

A Canthan character may quickly and cheaply improve their storage by saving up their Monastery Credits, using them to buy two Runes of Holding for their bags from the Monastery Quartermasters in Shing Jea Monastery. The cost is three credits per rune.

An Elonian character may similarly save up their Battle Commendations to obtain Runes of Holding from the Sunspear Quartermasters in the Sunspear Great Hall. The cost is three commendations per rune.

Trivia
Likely an oblique reference to the Bag of Holding from Dungeons & Dragons.