Last night, in our Pathfinder game, my intrepid adventurers were ready to cross the 400 foot bridge that spans the gap between crater and Castle Scarwall. The druid had already accidentally triggered a rain storm from the heavy black clouds overhead through casting the Lightning Storm spell. Yeah, I know the spell doesn’t normally do that but it just seemed right to have it happen. Gargoyles were whipping down towards them but being startled back with lightning bolts as they went. Anyway, when they were midway across, when a squad of enemies came out from the gatehouse and headed across the bridge to reach them.
The party waited at their point on the bridge managed to whittle down the enemy at their approach and were just mopping up the remnants by knocking over the skeletal nightmare when a shrieking roar sounded above them and a huge dragon with arched windows rose up over the bridge (it had sneak-flown around the castle from just above the lake and then landed on the bottom of the rather stoutly made bridge and then crawled along the edge towards them with its Hide in Plain Sight abilities going).
The dragon revealed itself and all but the Oracle, Guenmarcus, made their saves against its Frightful Presence. Of course, due to their level, even Guenmarcus was only shaken. It followed up its appearance with a Darkness spell to help it hide and then hide it did. Cue a battle of breath weapons, attempts to hide, and then all out attacks on the paladin.
Anyway, the point is that it felt more organic with this interruption to the regular routine than if the dragon had been in its appointed place awaiting their attack or even if it had ambushed them later. It worked out because the gargoyles were more of a distraction / environmental hazard that the druid managed to cope with for them. It was also important that the party had plenty of time to take out the approaching squad (which was all of low to very low CR to them) so that the interrupting dragon didn’t mop the floor with them. They weren’t low on spells. They weren’t surrounded by difficult baddies. They’d gotten a difficult cherry on top of the rather easy icecream.
And it worked out nicely!
In truth, this is just an article to say that this form of Combat Interruptus, when done well, can really boost the flavour of the game and add to the immersion value without damaging the ‘fun’ value like most elements of realism tends to do to the sword-and-sorcery fantasy genre.