There are those of the Mechanicum who decided to turn on the Emperor and follow Horus and his cohort in the pursuit of Terra, the cradle of humanity. As each of the traitorous Legios turned on the Imperium their mutations grew more gruesome; and nothing represents this better than the fallen Titans who adorn their engines with trophies, spikes or flesh. There are none who turn to Horus' path and return, from Conscripts to Princeps, any who pledge themselves to the Ruinstorm Gods do so with no chance of redemption.  

Even though this army goes against everything Smaugonite stands for with his hobby, (after all it isn't wise to turn your back on the Imperium!) the lure of the Chaos Titan kits got the better of him and he managed to salvage an unwanted Titan and bring it back to its full glory in service of the Dark Gods. 

In Smaugonite's Titans of the Ruinstorm collection*, there are the Titans listed below:

INCENDIUM ELMO (Warhound Class) #405

INNOMINATUS (Warhound Class) #7887

NOCTIS ASTRUM (Reaver Class) #2914