You must -

Have completed the lost city quest
Have 27 empty inventory space (This is for 26 essence and your cosmics)

For people who have never run before -

You come to the Cosmic altar in The Lost City, I trade you 26 noted essence, you run/walk to the bank and un-note the essence then bring it back to me. Per run you get 13 cosmics. This is basically FREE runes. I am ALWAYS in world 84, runescape name - L3mmings

If you're interested please post below with your runescape name.