A death rune obtained in the Runespan is created by siphoning death esswraiths (with 65 Runecrafting), skulls (also 65 Runecrafting), bloody skulls (with 83 Runecrafting) and undead souls (with 95 Runecrafting). Siphoning requires one rune essence and gives 60, 120, and 144 experience respectively.
Death runes are worth 2.5 Runespan points and can be used to cross skeletal platforms, greater missile platforms, flesh platforms and greater conjuration platforms. They will disappear and be converted into points if you leave the Runespan.
If you try to bank any runes or essence from Runespan you get the message "A magical force prevents you from banking this item."