Basically, we have Forge-Adept Validus Rooke, who's either a member of the Raven Guard from just before the 21st Founding, or a member of the Warp Skulls from just after the 21st Founding; maybe you want me to take up a Captain slot before using a character on the Forge-Adept, which I can always do instead of this, but I suppose multiple characters isn't an impossibility either if this one is allowed. Anyway, Validus gets sent off to study under the Mechanicus for a few decades, and comes back with an almost obsessive reverence for the Omnissiah even by Mechanicus standards, to the point that he's already half-machine by the time he comes back, and within a couple more decades has ended up replacing everything but his brain with machinery. The only reason he doesn't immediately create and install an artificial copy of his own brain is because he's near-constantly modifying his actual body to be ever more powerful and efficient in his free time, upgrading parts beyond Imperium standards and adding ever more servo-arms and mechadendrites to his servo-harness, never mind what he's doing to many of the chapter's servitors, weapons, vehicles, and sometimes even the serfs if they'll allow it, such that he never finds the time to just finalise his own mechanisation; in a normal chapter, he'd probably have been chastised or excised from service long before then, but the Warp Skulls have a close bond with the Mechanicus, so it's allowed for a fairly extensive period of time, given that he hasn't gone and created his own personal army of Skitarii just yet.

All that happens before the mutations start taking hold of the chapter's members. Validus Rooke's mutations begin at about the middle stage of the chapter's overall mutation, possibly because he's already gotten rid of most of his organic tissue; when they do take hold, though, they almost seem to be concentrated in his only organic part left - his brain. Tentacles emerge from his artificial skull, his thought processes are vastly altered, and where his previous personality was stoic and logical, if somewhat overenthusiastic, his new personality is quite deranged, as he claims to start hearing the Omnissiah itself talking to him in purest binary code, pushing him to perform ever more extreme modifications to himself and his charges (most notably, replacing the organic tentacles growing from his brain with even more mechadendrites), until one day, when whatever has taken hold of the chapter and his mind hits a critical point, and he abruptly and somewhat violently develops into a psyker.

This puts the chapter in an unenviable position of deciding what to do with him - whilst what he was doing before was barely acceptable, his new abilities seem like they'd be absurdly dangerous in somebody of his temperament and physical capability, and his psyche likely won't benefit from suddenly being an open pathway into the Warp either, if it's not already corrupted beyond redemption by whatever has happened to him; on the other hand, dealing with him through any means that isn't straight overkill may be inviable, and despite his madness, the man has continued to produce brilliant solutions to previously-unknown problems, to the point of eliminating the overheating issues with many of the chapter's plasma weapons. Eventually, it's decided that he'll basically be heavily brainwashed into a more acceptable state of mind, then trained further in containment and use of his psychic powers, since both those and his knowledge are too valuable to just outright get rid of; this happens, and his mind reverts to a more stable state, albeit one of moderate confusion around anything that isn't his relatively weird psychic abilities or the chapter's machinery, with his brain developing the crystalline coating normally present on a standard Librarian's skin as he's trained to utilise his new powers, both as offensive measures against the Warp Skulls' enemies and to better service the machine spirits of the chapter in spite of his extreme deviation from the Imperium's normal setups for their devices. Said deviation may or may not be part of the Warp Skulls' current predicament, but that's a discussion for another time.