A soul rune obtained in the Runespan is created by siphoning soul esswraiths, living souls (both requiring 90 Runecrafting) or undead souls (with 95 Runecrafting). Siphoning grants 106.5, 213 and 144/255.5 experience respectively.
Soul runes are worth 3.5 Runespan points and can be used to cross greater conjuration platforms. They will disappear and be converted into points if you leave the Runespan.
Players that try to bank any runes or essence from Runespan receive the message "A magical force prevents you from banking this item."