Then she said unto the tech-priests, "Let it have a very large chainsaw, for to give a righteous beating to the unbelievers. And let it also have a flamethrower, for to get medieval upon the asses of the blasphemous. And so it may lift up the hearts of the faithful, and remove the hearts of the faithless."
And there was much rejoicing and/or wailing and gnashing of teeth, as appropriate.
While I'm a big fan of the tanks and power-armoured Sisters of Battle, I'm less keen on the half-naked flagellant element of the army. The Penitent Engines, especially the new ones, always felt like a missed opportunity, and look more like crude and rusty torture implements than the ornate bits of sacred war-machinery than I imagine them to be. I see them more as bits of gothic masonry come to life.
I made the main part of this model ages ago. It uses a Hasslefree miniature as the statue in the centre, with parts from 40k and the Warhammer Garden of Morr (mainly gateposts) to bulk it out. The power unit at the back came from a plastic toy, as did the arms. The legs are from a Sentinel.
I think it looks suitably deranged, and will fit nicely alongside the other Relic Engine:
And the tanks: