Can i use a guard to interrupt monsters on Dire Warning?

Guard orders can only interrupt the overlord's turn. If a new area was revealed during the overlord's turn, then you could use a Guard order during the effect of Dire Warnings, but it usually triggers during a hero's turn, in which case you can't.

You can use a Guard order to interrupt the overlord when he plays Dire Warnings, but I assume that's not what you meant.

Ok thank you, that's exactly what I wanted to know ; )