It's mentioned at least a couple of times in the original trilogy that beheading was only mildly annoying to the Lord Ruler. What combination of powers would allow him to easily survive such an extensive amount of trauma?
It's clear that using Allomancy to burn pewter can make a person much more resistant to injury, but a number of people are killed while burning pewter over the course of the trilogy, most of them to lesser wounds than a decapitation. An injury to a vital organ that would have been instantly fatal to a normal person generally seems to be treated as still being instantly fatal to an Allomancer burning pewter.
And it's clear that Feruchemy can be used to rapidly heal severe but non-instantly-fatal wounds. Presumably this is (part of) how the Inquisitors are able to recover from wounds that should kill them. Although even an Inquisitor can be killed by beheading. It's even the preferred method for killing them, until the secret about the spikes is revealed.
And there seems to be some property of Hemalurgy that allows a person to continue functioning despite damage to normally vital organs (including the heart and brain). I don't think this is ever fully explained, beyond stating that it's some combination of Hemalurgic spikes that keeps Inquisitors alive which is why decapitation works against them (by separating the spikes in their head from the others). And in any case it did not seem like the Lord Ruler practiced Hemalurgy on himself (possible exception being the bracers on his arm, which it's mentioned pierced his skin so that they'd be more difficult for an enemy to affect using Allomancy).
The Lord Ruler was able to use both Allomancy and Feruchemy, giving a boost to the latter power. But I'm not seeing how that would extend to the point of invulnerability to decapitation. Did his bracers also work like the Inquisitor's Hemalurgic spikes?
So what combination of in-universe powers would allow a person to shrug off a beheading? And how would it work; would the head grow a new body (how could it, since all of the metals it would need are with the body?), or the body a new head (how would a headless body know that it needed to do so?), or something else?