The PCs are being chased through the forest by something big and dangerous.
This adventure starts en media res, right in the middle of things. And in this case, they're running (most likely for their lives) from a creature that they'll have to trick or trap or ambush in order to escape. And it's a creature that will track them down and end them if they're not clever enough to deal with it.
What the creature pursuing them is depends on the GM and why they're being pursued. The choices presented include a pair of land drakes or chimera or choose your own. Perhaps they've stolen the creatures' egg.
The PCs find themselves lost in the woods, within sight of each other, taking a quick break from running. They're miles from any town or manor. They hear pursuit right behind them, coming fast.
The creatures are intelligent and very wily. One will chase their prey directly, driving the PCs towards the other. They know which PC has the egg (by keen smell), so will avoid breathing on that person, planning on taking out the rest of the party one at a time then cornering the egg bearer for the final showdown. They will not negotiate and will follow the PCs wherever they go.
Land Drake; No. Encountered: 2; Alignment: Chaotic; Armor Class: 2; Hit Dice: 5d8, Hit Points: 38, 38; Attacks: Bite/Claw/Claw (1d8/1d8/1d8); Save: F8; morale: 12 (currently, normally 9); Special abilities: Breath Weapon (4/day; variable effect, changes with each attack, roll 1d6: 1-2 Fire cone, 2d6 damage, 3-4 Acid cone, 2d6+2 damage, 5 Gas Cloud, Save vs. Paralyzation or be stunned for 1d4 turns, 6 Lightning Bolt, 2d8+2 damage).
Chimera; No. Encountered: 2; Alignment: Chaotic; Armor Class: 4; Hit Dice: 9d8, Hit Points: 50, 50; Attacks: Claw/Claw/Bite/Bite/Bite (1d3/1d3/2d4/2d4/3d4); Save: F9; morale: 9; Special abilities: dragon head breathes fire (3/day, 3d6 damage, Save v. Breath Attack for half-damage).