undertake (v)
contract (v)

Definition: enter into a contractual arrangement

tackle (v)
take on (v)
undertake (v)

Definition: accept as a challenge
Example: I'll tackle this difficult task.

arrogate (v)
assume (v)
seize (v)
take over (v)
usurp (v)

Definition: seize and take control without authority and possibly with force
Example: He assumed to himself the right to fill all positions in the town. he usurped my rights. She seized control of the throne after her husband died.