Yeah so, you obtained the Elder Cipher. It is a exotic bounty with several steps, like every other exotic bounty. The weapons you could "choose" from were just a preview. To complete the Elder Cipher, you have to find the Queens Cipher first. You can obtain one in a Treasure Key chest at the end of the level 32, 34, and 35 Prison of Elders. It's not a guaranteed drop though. Then you have to give the Cipher to Variks. He will repair it and after a few days you can get it from him again. Check every day. Then you have to obtain 1000 points by defeating bosses in the Prison of Elders. Normal bosses, as encountered in the level 28 Prison are worth 75 points. The level 32 boss is worth 200 points, the level 34 is 350 points worth, and Skolas grants 450 points. When you have 1000 points, you're done! Return to Variks and then you finally choose one of the weapons he showcased.