Star of Eyes turn in - after Tarton's Wheel
 

After completing Tarton's Wheel, Sulgar give you Staff of the Wheel and Star of Eyes.

If you turn Star of Eyes back into Sulgar, he returns a Rune of the Astral to you. This rune does not stack with other runes and when trying to combine with Rune of Impetus returns a message saying that the combination does not exist.

Working as intended.

Astral rune from that quest can only be combined with Frost rune for Ice Comet.

The dropped rune of the astral is required for the Supernova quest.

Thanks Dald- can it only combine with the frost rune from turning in the staff to produce IC?

Correct.

Those no drop runes can only be combined with each other to create Ice Comet. Because they have different item IDs from their droppable counterparts. The game looks for item IDs when combining which is why it failed to combine for Supernova.