A Gate of Idramm is a rune that can call a powerful demon. The rune is used primarily for destruction and is one of the most dangerous runes of the Tortallan Universe.
Akhnan ibn Nazzir tried to draw a Gate of Idramm and would have completed it if Faithful hadn't gotten Alanna to stop him. If he had completed it, it's possible that it would've destroyed the entire Great Southern Desert, not just the Bloody Hawk village.
The Gate of Idramm was never mentioned after The Woman Who Rides Like a Man, and demons in general were not continued with in the Tortallan Universe. As it seems to be a relatively easy rune to draw (although incredibly destructive), as Akhnan would have been able to do it correctly despite being a weak mage, it seems odd that it never came up again—especially in conspiracies and plots where the goal was mass destruction. It is likely that Tamora Pierce nixed the rune's existence—like she did with demons.