  We study existence, uniqueness and boundary blow-up profile for fractional
harmonic functions on a bounded smooth domain $\Omega \subset \mathbb R^N$. We
deal with harmonic functions associated to uniformly elliptic, fully nonlinear
nonlocal operators, including the linear case $$ (-\Delta)^s u = 0 \quad
\mbox{in} \ \Omega, $$ where $(-\Delta)^s$ denotes the fractional Laplacian of
order $2s \in (0,2)$. We use the viscosity solution's theory and Perron's
method to construct harmonic functions with zero exterior condition in $\bar
\Omega^c$, and boundary blow-up profile $$ \lim_{x\to x_0, x \in
\Omega}\mathrm{dist}(x, \partial \Omega)^{1-s}u(x)=h(x_0), \quad \mbox{for all}
\quad x_0\in \partial \Omega, $$ for any given boundary data $h \in C(\partial
\Omega)$. Our method allows us to provide blow-up rate for the solution and its
gradient estimates. Results are new even in the linear case.
