Ground astral rune is a quest item used in both the Dream Mentor and Dragon Slayer II quests. It is created by using an astral rune on a hammer, and subsequently using the resulting astral rune shards in a pestle and mortar. It is an ingredient in the shared dream potion needed in the quests.
You may have to visit the Oneiromancer to get a new Dream vial (empty) and have it in your inventory in order to break the astral rune with a hammer.
It has no other use thereafter, however.
- Ground astral rune is one of the two magic-imbued runes that can only be ground by pestle and mortar, aside from mud rune.