No one guessed correctly. The herb was tormentil! I might have spelled it incorrectly, but oh well. Here goes the poem:

The most scented of them all,
I shall make you drool and bite.
For I was found by a kittypet,
First used on a rogue. (Known to Warriors History)

Also, this book is becoming like a dead book. I'd like to continue it, so please help!
~Night