
Just want to make a few quick points here. First, I think this card is somewhat underappreciated. Three resources, a card, and an action to automatically dispose of a Hunting Nightgaunt or Conglomeration of Spheres is a great deal. It's a shame that this is a Seeker card, because Seekers have some of the most valuable resources and actions in the game. If this were a Rogue card, it would be much better. Still, there's nice value here.
Remember that reaction triggers are optional (so you don't have to throw away your Disc if you draw Swarm of Rats), and that you can use this to discard a teammate's draw (provided you're at their location, which you should always try to be), so getting good value out of this is pretty easy. Unlike something like Tooth of Eztli, you can get full value out of the Disc even if you draw it in the mid-to-late game.
There's a nice interaction with Scavenging. I don't really recommend this for Rex (he has far, far better things to do with his actions and money), but I think this is a pretty reasonable build for Minh.
Finally, this card gets much better in the Forgotten Age campaign. Ordinarily, the fact that this card discards rather than defeats the enemy is a drawback. That means it doesn't add victory enemies to the victory display, reducing the number of attractive targets. In Forgotten Age, though, Disc of Itzamna is an outstanding response to big Vengeance enemies like Boa Constrictor.