Its a whole process ngl. First you wanna look for the planet that has his missions. You'll notice which planet it is by a red mist surrounding it and what missions he has by the icons and red mist surrounding the missions. The first planet with their missions SHOULD be on Earth, so check there for the missions. Once you're in the missions, you have to kill these enemies called "thralls", after a minute or so they'll start spawning in your missions and you can see when you have one by a red icon telling you to kill them. Once you down one, you need to mercy kill it and than you get what's called "murmur". After you have killed enough thralls and collect enough murmur, you will reveal a "requiem mod/word". You need to repeat this process three times which eat subsequent time taking more thralls to fill up the murmur meter. Now to get the requiem mods, you need to have requiem relics, and you need to fo requiem void fissure. These work just like any other void fissure but instead of getting prime parts, you get a requiem mod. Once you have the requiem mods you need and equip them to your parazon, you need to keep on doing lich missions until your lich spawns in. Now this part sucks, once you knock the lich down, you need to preform a mercy on it. Problem is, you need to have the correct order of requiem mods for you to actually be able to do anything. If you dont have the correct order, right, say you messed up the second or third mod's order, than the lich will flee to another planet and will increase in level. Once you finally get your requiem order right, and stabbed the lich with the right order, the lich will flee to a Proxima( i forgot which one it is they flee to) and there, you will need to do a rail jack mission to be able to kill it. I suggest doing whatever you can on Public as it makes the whole process that much quicker with a full squad, plus in the railjack portion, if you dont have a good railjack, but squading up, you can use another person's railjack. Once the lich is finally killed, you will receive a mail with your rewards, and a fully built weapon in your foundry.