There is no such single wrapper dll.

K2 ships with a number of APIs.  Generally the most commonly used are:

- SourceCode.HostClientAPI - to build out K2 connection strings

- SourceCode.Workflow.Client - workflow interaction (start a process instance, action a task, retrieve a worklist)

- SourceCode.SmartObjects.Client - to interact with SmartObjects

I'd recommend referring to the K2 Developer Reference that installs with the K2 documentation.  It has lots of information and code samples around K2 APIs.