  Magnetars exhibit a variety of transient high-energy phenomena in the form of
bursts, outbursts, and giant flares. It is a common belief that these events
originate in the sudden release of magnetic energy due to the rearrangement of
a twisted magnetic field. We present global models of a 3D Force-Free (FF)
non-linear twisted magnetar magnetosphere. We solve the FF equations following
the Grad-Rubin approach in a compactified spherical domain. Appropriate
boundary conditions are imposed at the surface of the star for the current
distribution and the magnetic field. Our implementation is tested by
reproducing various known analytical as well as axisymmetric numerical results.
We then proceed to study general 3D models with non-axisymmetric current
distributions, such as fields with localised twists that resemble hotspots at
the surface of the star, and we examine characteristic quantities such as
energy, helicity and twist. Finally, we discuss implications on the available
energy budget, the surface temperature and the diffusion timescale, which can
be associated with observations.
