Just keep in mind that in order for the Avatar to die from one hit, he'd actually have to be facing his own race (silly, that is), facing off against a high-cost or special character (pretty likely if you don't play it safe), or playing him in an APOC game (which, oddly enough, is the best theatre for him fluff-wise).
Generally, keep him away from Librarians and Wraithguard and you should have no worries.
Just so it's clear, Force Weapons do *not* inflict 'Instant Death' they simply slay outright. Never is the Instant Death rule invoked (I know it sounds petty, but it's an important distinction. Names of rules are capitalized for a reason) This means Abby and DPs *can* be killed by force weapons.
Now, with that said, the Wraithlord is *not* immune to Instant Death, but since no S12 weapons exist, there's only one thing I know of that can inflict Instant Death upon him (Not counting Force Weapons which,as stated, work differently)...The Blissgiver.
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