Pusillin is an imp mini-boss residing in Dire Maul East. Pusillin stole Azj'Tordin's book of incantations. His initial location is near the main entrance to the instance. Talk to him, and he skips away.

Where can I find Pusillin?

To get keyed: Talk to him, he runs away, catch him, talk to him again.. rinse and repeat until he runs up to a balcony alone in The Hidden Reach area of the zone. Find him on the balcony, talking to him will spawn five (5) fire imps.

Can you get crescent key solo?

The Crescent Key for DM North and DM West can be obtained with a quick jaunt into DM East. Although geared/skilled rogues or druids may be able to solo this, it's recommended to bring a group.