I haven't been on zOMG since you could level up past 10. My question is how do you level up past 10?
⇢ Monsters in Deadman's Shadow drop dark blue orbs called shadow orbs which you use to level from 10.1-12.0.
It requires a crew and assigned ringsets instead of wearing whatever you want like previous areas of the game though, so read up on some guides first if you plan on attempting it.
As everyone in this thread has stated, you can get shadow orbs from Dead Man's Shadow. It's located in Dead Man's Pass, past where you found Stoochie. Nymph of Sprint wrote a great guide for it. If you need a crew, be sure to check out Rainbow's DMS Guidance Thread.