See, drawing inspiration from folklore and legend, when an ancient evil is left in a magical sleep under a swamp, you don't go and wake it up. That ancient evil has been dealt with, move on the to the next contestant. It's like toxic waste; nobody wants it in their backyard, but once it's sealed up and contained, you leave it alone...
Whos to say that the ancient dragon didn't have a clone stashed away somewhere? Or, something to consider, now that it's been released from its' magical sleep, a True Resurrection is all it takes to bring it back to full power. While it was magically held under the swamp, it was neutralized. Now that it's just dead, it has a near-limitless amount of options!
(as an aside, you want to be careful with this kind of thing: Resurrecting a defeated villain you weren't done with after a fair fight can look cheesey to the players. They might feel their voctory is lessened by the resurrection. But, given that they took out this dragon as a side-trek, with no backstory leadng up to it, in a 2-round fight, I don't think you'd be taking too much away from them...)
Whos to say that the ancient dragon didn't have a clone stashed away somewhere? Or, something to consider, now that it's been released from its' magical sleep, a True Resurrection is all it takes to bring it back to full power. While it was magically held under the swamp, it was neutralized. Now that it's just dead, it has a near-limitless amount of options!
(as an aside, you want to be careful with this kind of thing: Resurrecting a defeated villain you weren't done with after a fair fight can look cheesey to the players. They might feel their voctory is lessened by the resurrection. But, given that they took out this dragon as a side-trek, with no backstory leadng up to it, in a 2-round fight, I don't think you'd be taking too much away from them...)