Life Drinker (Although Not Really): Not in any literal sense, obviously, this being firm SF and thus vitalism very much not in vogue. But consider the case of the experience addict, who eats forcibly taken memories, or those who take an individualist approach to group-mind transcendence by attacking others to forcibly merge their victims mind-states (pithed or complete, although the latter is a swift path to crazy) into their own. (And, hey, you might as well end up in the younger body while you’re at it, right?)
It ain’t the same thing, but it’s close enough for metaphor.
(And if you were wondering, yes, soul-eaters – which consume your mind and memories and individuality and capacity for choice – are what Eldraeic vampire myths look like.)
Legendary Weapon: In the Empire, the thing about most legendary weapons is that they tend to stay legendary… due to not staying the same weapon, or rather, the same embodiment of the weapon.
Aorillia, the Sword of Illimitable Light, for example, the legendary weapon of the champions of the Solar Empire, has over the course of its history been three swords, two sniper rifles, a man-portable laser, three different battleships (one wet, two space) and a dreadnought. (Many of the earlier examples of which are, indeed in display cases in museums.) But they share the name of the legendary weapon, and theologically speaking, they share the essential spirit of the weapon – and so for all mythographic intents and purposes are fundamentally the same weapon even if their materials forms and capacities are obviously not.
And since the mythographic truth is the important thing when it comes to a legend, thus it is.
Language Drift: Sort-of averted for Eldraeic, due in equal parts to its origins as a designed language intended to communicate precisionist-grade thought and to its ongoing tending by the logotects, et. al., of the Conclave of Linguistics and Ontology, who are prescriptivists nonpareil. Only sort of, however, since their department of Worthy Innovations routinely combs the language as it is spoken for, well, worthy innovations to be taken up into the canonical version.
Since they’ve been doing this for a long time, and since almost nothing can ever be thrown out due to the obvious need for backwards compatibility in language design, the result tends to be — well, if it were English, they’d be like this (courtesy of xkcd):
Played rather straighter by most other languages of the Worlds, although both the influence of Eldraeic via Trade and that of the pervasive communication networks of starfaring cultures do tend to slow it down a bit.
Killed to Uphold the Masquerade: Again, a tactic used primarily by lower-tech and lower-ethics organizations, ones which can’t manage or don’t bother with amnesic drugs, memory redaction, a geas, simple memetic discrediting, or if all else fails, a trip to a perfectly nice, perfectly comfortable luxury space resort that no-one ever leaves.
Kill It With Water: Leaving aside such not-really-the-water examples as water knives, pressure effects, and drowning, this is rather rare, inasmuch as the universe is full of the bloody stuff and as such having much of a vulnerability to water is unlikely to let you live long enough for anyone to actually try to kill you. (And any species having one would be unlikely to be so freakin’ stupid as to invade a small sylithotectonic world whose surface is something like three-quarters covered in the stuff. Yes, I’m looking at you.)
Perhaps the best known related, not-really-the-water-but-more-so-than-some, example exists mostly because of those species whose native temperature range is such that water’s natural state, as ice, is a kind of rock. And being hit by a stream of liquid water, therefore, is much like immersion in lava.
Kill and Replace: A tactic – and with modern sophotechnology, a lamentably obvious tactic – used by lower-tech and lower-ethics intelligence agencies, for whom fast-growth cloning (or even radical plastic surgery) and programming is the best they can manage – rather than the implantation of overshadowing gnostic overlays used by the technological sophisticates among shady organizations.
Jump Jet Pack: A ubiquitous addition to virtually all combat armor (see: light infantry), scout armor, and the equipage of just about everyone else who can benefit from high mobility, from disaster relief teams to microgravity construction workers.
(Being able to bend momentum with your mind is nice, but some rocket thrusters sure help.)