The bone ladder is used to access the upper level of the Runespan from the middle level. The ladder requires level 66 Runecrafting to use and is located in the south-western corner of the middle floor in the Runespan.
Temporarily boosts work for this requirement, so it can be ascended at as low as 59 Runecrafting (with a random Advanced pulse core +7 boost), or 61 Runecrafting (with super runecrafting potion's reliable +5 boost). However, the lowest level creatures and nodes exclusive to the top floor are Death esswraiths and Skulls, both requiring 65 Runecrafting.
A map of the middle level of the Runespan. The bone ladder is on the island numbered '40'.