I've been requested to post my Mated Dragon Encounter. This is a battle that I created way back in the 90s. It was written with AD&D 2E in mind, so the abilities are from that era. I don't have the exact specifications with me, so I'll write it up from my aging memory....
This encounter is for a group of at least level 10 or more. Preferably higher.
The setup: The party has been exploring a cave system, fighting all manner of beasties. A cave-in has forced the group to press on, unable to return the way they came, and hope for another way out.
DM: As you continue down the corridor, you feel a draft coming from ahead of you. The corridor starts to open up and you see before you a vast cavern. a rocky bridge spans across a massive cavern. This cavern is so big, you cannot see the sides of it. it is at least 500 feet wide to either side of the bridge. The bridge is sturdy and on the opposite side of the bridge you see faintly, two torches on either side of an opening. The bridge does not have any sides, and is only 10 foot wide. You peer over the edge and vertigo grips you and you step back. You cannot see the bottom. Also, looking up, you cannot see the roof. Knowing that you cannot turn back, you realize that it's going to be a treacherous walk across the bridge that will have to be slow and steady. As you get about halfway across the bridge, you notice that the stone is blackened. A soot coats the bridge. Suddenly the world goes dark.
DM NOTES: The cavern is 500ft wide to either side of the bridge. torch light won't reach the edges. The bridge is 300ft long. it will take 10 rounds to get across if a slow steady pace. if not, reflex saves will need to occur to avoid slipping and falling off the bridge. The bridge is also 200ft high. the bottom of the cavern is laced with "spikes". Stalagmites stick up, and there is a high chance to hit them if you fall. The ceiling is 300ft high from the bridge. Stalactites lace the ceiling. The floor of the cavern slopes upwards towards the edges, similar to a bowl. At the 450 mark from the bridge on either side is an area that flatter and is large enough to hold a dragon. In fact, a mated pair of mature adult dragons perch on either side of the bridge.
The dragon's eyesight can see to the bridge. Once a group of adventurers get to the middle of the bridge, one dragon casts darkness onto the bridge, not on a character. this avoids the save of the spell. The other dragon casts fireball of 9th caster level at the bridge. They then start flying towards the bridge, they will get there in 3 rounds and land.
Any party member caught within the darkness will have to make reflex saves to avoid being disoriented and falling off the bridge. Once the fireball hits, the disorientation is worse. The dragons land, each one on opposite sides of the darkness spell, basically shutting off any chance of escape. The landing is forceful, reflex saves to avoid falling down from the bridge shaking. Anyone that has escaped the darkness is confronted with a dragon, and the darkness prevents the character from seeing the other side of the darkness, and the other dragon. if no one has escaped the darkness, both dragons will breath once on the bridge, then the one that cast darkness wills it away. to see if their dinner is done cooking. The dragons will fight, but will use tail slap to knock the enemy off the bridge. anything to minimize damage to themselves and have a quicker dinner.
Yea, this is considered a TPK style of encounter, hence the party needing to be of a very high level.