After grinding my ass off the last few hours, Strikes and the cave and whatnot, I got a couple legendary engrams, on my lvl 24 Titan.
Side note: Just before that, I had been playing my Warlock. Level 10.
Noticed while I was grinding that I got a grimoire card for 5000 fallen kills with a 'lock.... While playing my Titan.
Didn't think anything of it.
Flash forward to now, at the Cryptarch, decoding my engrams into lvl 20 Warlock class legendaries.
THE -blam!- BUNGIE?!
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Legendaries are random class. Says in the disclaimer of the engram.