Basic intro: 
For one reason or another, each of the PCs owes a significant debt to the Dwarven Mafia.  They have elected to pay the debt off through work, rather than having their rib cages crushed.
The boss in your ward is Radovan Ribcrusher, an up and coming Dwarven crime lord.  He has promised you that if you will work for him as "Rat Catchers" (urban monster hunters), you will pay off the debt in no time while still earning something for yourselves.  He has a number of jobs lined up, and no doubt you will be able to find even more work besides.  It's all about hustle.

Example character hooks:
You borrowed money to:
- start an armor supply store, which failed badly.
- start a halfing strip club, which failed badly.
- pay for medicine for dear sister/mother/brother
- cover gambling debts
- cover the losses on your last merchant voyage.
- pay for bribes to get friend/lover/family member out of jail
- pay bribes to get friend/lover/family a position in the ministries

- You were given over by your community as payment, in lieu of protection money
- You were was too blood thirsty for fighting in the blood pits.  It's just an act man, follow the choreography, no one is supposed to actually die.
- You are an agent of  Gloomtail's secret police, ordered to gather information on Dwarven Mafia operations.
- Your entire family died of plague, and you now want a simple job where you can meet your death honorably
- You come from a line of monster hunters. Your father and his father before him managed to have children before dying on the job.
- You are a noble, slumming it.
- You are a noble and a dilettante writer, gathering material for your next book
- You Uncle is a higher up in the Dwarven Mafia, and had you assigned to this job to prove your worth before moving on to more responsible positions
Example starting stories for a Far-Explorer:
- Scion of a noble house who can afford attuning and implanting an Astral gem.
- Confidence man/Forger, who impersonated being a member of a House well enough to get the operation
- Adventurous scion of a trading cartel
- A secret agent of one of the Dragon Lords, funded in order to make sure nothing useful or harmful slips by
- A known representative of one of the Dragon Lords, e.g. an army captain, a junior member of the Scholomance, non-so secret member of the secret police, etc. 
- Lottery winner
- House breaker who hit upon a grand score and decided to turn it into a chance for even larger treasures
- Fortunate merchant captain, sold his ship and profits for the chance
- Chosen by subscription from human worker slums
- Gnomish terrorist funded by the Gnomish Liberation Front.  Disguised as a halfing.
- Academic chosen for actual knowledge and qualifications in exploring a new world.