This is a conversion that uses bother captain sterns body with ahrimans head and staff, with a plastic chaos power fist. What you don't see is the combi-bolter made from 2 thousand sons bolters on his back.
The mini looks good, I wouldn´t have guessed that it´s based on a Grey Knight´s body, but it fits really good with the Tzeentchian parts. Compliments to your conversion. The painting also looks good, your highlighting looks nice especially on the helmet, the blue/yellow decoration on his horns is also well done (I´ve once tried to paint a thousand sons headpiece, but your paintjob is definitely better).
Only the base is a bit too blank. Perhaps a bit sand or grass would approve the overall look of this mini greatly, but otherwise, it´s a fine mini and a very good conversion.
Please visit my Chaos minis too.