The Khvostov is the first gun you get in Destiny, and though it's unlikely you still have the underpowered gun by the time you play Rise of Iron, you can recapture that magic again by constructing the Khvostov 7G-OX, one of the Exotic weapons you can find. Here's how you get the Khvostov 7G-OX.
For more tips and hints for Destiny: Rise of Iron, visit our Destiny: Rise of Iron Guide Hub.
Start a Patrol in The Plaguelands and go to The Breach, which you can get to by going to the Rocketyard from Lord's Watch, then taking the branch that leads to The Divide and keep going until you make it to The Breach. Continue towards The Cosmodrome – the first place you woke up in at the beginning of Destiny – until you get to a gate that can be unlocked with a Splicer Intel Relay, which you can find as an uncommon random drop for killing Brood Mothers. For more details on finding this and other random drops, look at How to Get and Use Splicer Keys, Splicer Intel Relays, and SIVA Offerings. Go through the gate and head towards the ship at the other end of The Cosmodrone. Kill all the enemies on board and look for a loot chest in the rear right corner of the ship. Open it to get the Khvostov 7G-OX schematic.
Take the schematic back to Shiro-4 at the Iron Temple to get the We Found a Rifle quest, which tasks you with finding 10 Manual Pages and three Khvostov Parts in The Plaguelands. The Manual Pages are easy enough to find, as enemies you kill during your patrol have a chance of dropping them. Chances are you'll find enough Pages by the time you find the three Khvostov Parts.
Unlike the Manual Pages, the Khvostov Parts are in set locations. Check out our guide How to Find the Khvostov Parts for the exact locations of all the parts. Once you've got all the Pages and Parts, talk to Shiro-4 again to get the mission Khvoslov Rising. Start the mission and follow the prompts until you get to The Divide, where you'll need to kill Kaviss, Splicer Captain. Your first encounter with him will be timed, and once either the timer runs out or his health gets low enough, he'll teleport away.
Tracking him down by following the SIVA Swarms he leaves behind, which look like black-red statues with red clouds of tiny insect-like things hovering around them. You'll finally corner him at The Breach for a final confrontation, this time with dropships spawning troops and servitors endlessly until you kill him. He's tough, so make sure you've got a Void weapon to take out his shield. And if you're still having trouble, bring a friend to help with crowd control, as staying alive with that many enemies spawning is really difficult solo.
Once you kill Kaviss, he'll drop the Khvostov 7G-OX, which you'll then use to storm The Cosmodrone and take out the enemy force on the docked ship there before sharing a nostalgic moment with your Ghost.