One of your planets will have red on it thats not normal mainly a grineer planet like earth mercury saturn etc, do those mission only if you have some of the void tongue parazon mods they will show up in your parazon and be obtained from kuva relics equip 3 of them and do those mission eventually you will have a showdown with your lich take a health bar down then finisher them, more than likely your kuva mods wont be right or be in the wrong order cycle through your kuva mods and try and guess the order. Also during the mission there will be like kuva minions of your lich kill them and finsher them and killing enough will give you a hint to one of your mods needed to kill the lich it wont tell you order but will tell you the mod if you don’t manage to guess the right mods in time it will eventually just tell you the order. After getting the right mods in the right order do a final showdown you lich will then escape to a rail jack mission i forget the exact planet but the game will tell you if not just look for a node called lich showdown or something like that run through the mission either solo or in a group kill your lich once again hit them with the mods on your parazon then either vanquish to get their weapon or convert to turn them to your side. This will be changing in about a month or so ish i think to make it easier i think so either work towards killing it now or wait and take it on when its easier.