Rune lock
The rune lock is a barrier found on the entrance to the Vaults, preventing you from entering that branch until you've acquired at least one rune of Zot.
Rune Availability
The only runes which are not available prior to entering the Vaults are the silver rune (found on Vaults:5) and the golden rune (found on Tomb:3).
Strategy
When hunting for your first rune, the variable Lair runes are usually the easiest to obtain. Followers of Jiyva can easily acquire the Slimy rune, and followers of Lugonu have a somewhat easier time acquiring the Abyssal rune. The Hell and Pandemonium runes are extremely difficult to obtain at this point in the game regardless of character. Note that entering the Vaults is not required to win the game, but optimal play will lead to it.
History
Although the concept of a rune lock had been debated since very early versions of Crawl, it was only implemented after the 0.13 tournament, in which acquiring a rune before D:14 was an optional challenge conduct. Prior to the rune lock, players had little reason to explore the Lair branches until after they'd explored most of the main Dungeon, leaving them very overpowered for the challenges the branch endings provided.
