Double Damage and "Instant Death"

 Empire of the Petal Throne (1975) contains the following section:

This is the first appearance of what would later be called a "critical hit" in the history of roleplaying games. Since the start of my ongoing House of Worms campaign, I've made use of this rule without modification. According to one of the players, there have only been four such "lucky hits" (as Professor Barker called them) in the nearly seven years we've been been playing, all of which affected opponents of the characters – until our most recent session.
During Session 253, the characters were exploring a series of caverns beneath a ruined step pyramid they'd found on a coastal island. There was plenty of reason to suspect the caverns were inhabited, not least of which being that they seem to have been picked clean of anything organic. This worried Kirktá, the apprentice to Keléno, scholar priest of Sárku. For that reason, he volunteered to keep watch on the ledges of a large cave while his comrades explored nearby. His worries proved well founded, as a large insectoid creature began to crawl down one of the ledges, apparently attracted by the echoes of the characters' actions.
Aíthfo and Grujúng rushed to meet the creature, attacking it as it slowly descended the wall of the ledge. They soon realize that its carapace protected it well and that, owing to its size, it would take a great deal of effort to slay it. Initially, the fight went well, with the creature failing to land a blow on any of the characters. However, on the third round of the fight, I rolled for a 20 and then a 19 against Aíthfo, resulting in his instant death. I decided that the creature's mandibles sliced through the unlucky Aíthfo's neck, severing his head from his body. 
Needless to say, this shocked everyone. No character had ever suffered an instant death due to the critical hit rule before. Ironically, Aíthfo had failed a saving throw some years ago that had resulted in his death, but he was eventually restored to life by Naqsái magic (which led to some long-ranging consequences). Now, though, the characters were quite far from any means of revivifying Aíthfo and worried that this might indeed be his end. Znayáshu, however, had an idea. After sewing his decapitated head back onto his body – Znayáshu is an accomplished embalmer – he made use of his excellent ruby eye on Aíthfo's remains. This device of the Ancients freezes its target in a moment in time. In this way, the body would be immune to decay or corruption until Znayáshu used the eye on it again. The body was then submerged in the water of an underground river to keep it safe.
The characters continue to explore the caverns. Once done, they plan to seek out some means of revivifying Aíthfo and will return to the caverns to do so. How or when this will occur is still unknown. Given the way this campaign unfolds, it could well be many, many more sessions before it comes to pass, assuming it ever does. But that's the nature of this campaign: it's unpredictable. In the meantime, Aíthfo's player has taken up the role of Lára hiKhánuma, a sorceress of Ksárul and a relative of Aíthfo's new wife (or should I say widow?). It will be fascinating to see what happens next. Aíthfo in happier times