I have always thought of having an grey knight army that was lead by an inquisitor. well this is the fluff:

Inquisitor Harris Amran leads an elite force of Daemonhunters that hunt down and kill any rouge inquisitors that have fallen to chaos. Now his crusade has left thousands of people dead, as he searched for the man that betrayed him a century before. His original partner in this force was Iquisitor Gallian Farmin. When the force thwarted an atempted summoning of the Khorne daemon prince Golihal, Gallian trpped him and his two most powerful daemons in the bodies of three of his oldest friends, and still to this day feels loyalty twords them. Gallian began to consort with the daemons, and was eventually seduced to the ruinous powers of chaos. Gallian prepared a ritual to summon the daemons back to the material relm, but was stopped when Inquisitor Harris interupted. the two fought while the grey knights with Harris stopped the outflow of daemons from the warp portal. Gallian escaped through the warp portal, but not without losing an arm to Harris' force sword. Harris then vowed to hunt down Gallian and finished what he started, and with half a company of Grey Knights and the force of the Emperor's holy Inquisition behind him. They followed Gallian's trail but the former inquisitor of the Ordo Malleus hides behind his cults and uses his Daemonhosts to hold off the forces following him. the two Inquisitors will probably fight on for the rest of their lives, as the forces of chaos are forever plotting ways for the harvesting of souls and the Ordo Malleus as always will be there to stop them.

i know this is kind of cheesey but it is something i thought would be awesome to write fluff for. i will probably make an army for this in the distant future but for now i just am making the fluff. to clarify something: this force was created BEFORE the treason and is one of the Ordo Malleus' most secretive forces.

To summarize, Harris' partner inquisitor Gallian betrayed him to the powers of the warp and Harris wants to kill him. Gallian hides beind his servants so one of the two will die befor this is over, weather of old age or by the sword.