The sinister key is dropped in yannile dungeon by salaran the twisted. It is used to open the sinister chest which contains 9 grimy herbs.
This guide will focus on buying the keys from the GE and using them to open the chest for profit.
http://runescape.wikia.com/wiki/Sinister_key
Here you can find the difference in price between the sinister key and herbs received from the chest.
To find the profit you will be making per hour just multiply the difference by the number of keys you're using.
Here are the requirements for chest running:
49 agility - required to access the chest in the yanille agility dungeon
Completion of the watchtower quest - This unlocks the watchtower teleport which puts you just outside the agility dungeon.
Money for keys - At around 40k each key they don't come cheap and if you want to make a decent amount of money you should already have 6mil +
This is what your bank should look like only hopefully with more supplies!
Sinisters keys
watchtower teleport runes - only take enough for one teleport
rings of dueling - for fast banking
super energy potions - only take 1 or 2 dose potions so you can drop the vial
super anti poison- The chest will poison you after using it, again 1 or 2 dose
Equipment
An equipped ring of dueling
A slashing weapon for the spider web guarding the agility dungeon.
boots of lightness - as long as you are 0kg or below it's fine
I can complete roughly 45 chest runs in 1hour resulting in 135keys used this gives a profit of 1,054,755 (45*3*7813) at current prices.
You can also use a beast of burden to increase the amount of keys you can carry
With a spirit terrorbird you can carry 4 keys with space for herbs.
Resulting in 1,406,340 (45*4*7813)
With a wartortoise you can carry 5 keys with space for herbs.
Resulting in 1,757,925 (5*45*7813)
With a packyack you can carry 6 keys with space for herbs.
Resulting in 2,109,510 (6*45*7813)
Keep in mind that using a beast of burden and therefore more keys will slightly increase the run time.