It's not impossible as I have one, but i have no idea how i got it. I think it was a chest engram from Xur some time ago, but i've never seen another one. I have two warlocks and one hunter and none of them have gotten these from a dropped engram, yet I get Titan gear all the time :( I have found that if I give my lesser played lock or hunter I have a higher chance of getting gear they need. If i buy an exotic engram from Xur and use my highly played warlock to decrypt it, i almost always get Titan gear.