The Caplain Secret Recipe was a magical item given to Lahni Caplain in The Elite Forge. It channeled the spirit of the Caplain family long passed, and would allow the user to harness their speed. Using it, Lahni could double her speed, and cause bursts of lightning to shoot forth and damage her enemies.
It's appearance was that of an old, worn-out index card, and written on it was the recipe of the Caplain's prized Ferret Pie/Cake (it could function as either a cake or a pie).
It's a little strange that the recipe could make a pie or a cake, since the reason the Caplains died all those years ago was a pie/cake family feud, but hey. Maybe they learned their lesson
The card would become bound to the user once read, and couldn't be used by anyone else unless it was passed down to them
Lahni could use the card to double her speed once per encounter, at a max of 3 times/day
Any attacks she does while her speed is doubled deal an additional 8d6 Lightning damage, if the attackee failes a DEX saving throw (essentially, functioning as the spell Lightning Bolt). Half damage on success. Also lights flammable objects on fire that aren't being worn or carried
When she does this, she must also roll a flat d20. If she gets a Nat20, time stops for 1d6 rounds! She can still move and takes actions as normal during this time
Gonna run some numbers here. If Lahni's speed is doubled, she can move 1000 ft/round. If she rolls a 6 on her d6, and moves in a straight line for all 6 rounds, time has stopped, meaning that in the span of a single round, she will have moved 6 rounds worth of distance, which, for her, is 1000 ft. This means that, in a single span of 6 seconds, Lahni will have traveled 6000 ft, or 1000ft/sec or 681.8 mph!!! For reference, the speed of sound is 767.2 mph, so Lahni is nearly travelling at Mach 1. Holy Flash!