The unfinished astral rune is obtained as a Glorious Memories quest reward from Nial Swiftfling. The player picks it up after defeating Nial Swiftling. After the quest, the player can bring it to Baba Yaga to receive the Prophecy Tablet and an antique lamp which gives 2,500 experience in any skill above 35. Between the dates of 2 March 2010 and 11 October 2010 after handing the rune to Baba Yaga, she would perform the Blow Kiss emote. After she did the emote the player would have to talk to again like she had never received it.
If you happen to lose the unfinished astral rune before giving it to Baba Yaga, you can talk to Chieftain Brundt about Glorious Memories to get it back (option 3. Ask about the birthday feast).