So I've played I believe 7-8 *350 Light Strikes and a Nightfall. And I have received ZERO of these so called Skeleton Keys. With friends I played with during these strikes only received one during that whole period of strikes. I don't know if my luck is just bad or if the drop rate is really low.
Edit: Thank you guys (and gals) for all the feedback!
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Agreed, skeleton keys are way too rare even though I get them every 10-15 strikes ...