You will find Task Masters generally, but not always, near bind stones.
They will say taskmaster above them, and by clicking on them you will given a task to kill a paticular mob, either a single one, multiple or all in a nearby cave.
If you are a caster, you would choose the long corridors, if your a fighter, the other.
You can do these alone or with a group.
You get progress reports in the dungeon (caves), by seeing how many more monsters you have to kill.
You will get money and exp for each kill and rarely something may drop.
If you are completely successful...you will get more exp and money.
If you are killed, a tombstone appears outside entrance, which you may pray at to get some exp back.
If you leave the cave before you are finished you fail.
If you run out of time, you fail as well.
When you become to experience for a paticular task dungeon,m the taskmaster will direct you to the next NPC.
Hope this help.