Acquire a new rat companion
From Fallen London Wiki
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From: Alliance with the Big Rat
For a price, the Big Rat will assign you one of his most feisty lieutenants, a Rattus Faber Bandit Chief.
Game Instructions: You can acquire Antique Mysteries by using Elder items in your inventory.
Unlocked with 
15 x Antique Mystery
Locked with 
1 x Rattus Faber Bandit-Chief
Success
Trussed and angry
The rat on your doorstep is bound tail and paw. When you pick the twine knots free, he bites your hand. You remind him who sent him and he settles down. In a day or two, he even accepts instruction. [...]
You now have 1 x Rattus Faber Bandit-Chief
Wounds is increasing… (+1 CP)
You've lost 15 x Antique Mystery
