Requiem Mods and Requiem Relics are new additions to the game, introduced with Update 26, The Old Blood. They are the only thing that can kill your Kuva Lich and must be used in the correct combination on your Parazon to kill the Kuva Lich using a Mercy Finisher. While normal weapons and your teammates can damage him, Lich's will be weak against a certain combination of mods, and only a hit with that combination can kill him. To know the combination, you need to kill Lich Thralls with the Parazon.
Warframe - How to get Requiem Relics and Requiem Mods
Requiem Mods, listed below, come from new Requiem Relics. These new Relics can be obtained from Kuva Flood as a guaranteed reward and from Kuva Siphons at a 30% drop rate. They are then refined in the Void Fissure missions of Kuva Fortress. These are the only relics that can be refined on Kuva Fortress.
Once refined, Requiem Relics open and can drop a Requiem Mod for your Parazon. There are four types of Relics, I, II, III, and IV, which appear on a new screen in the Relic menu.
Requiem Relics can also drop an item called Slivot Riven. These Riven Slivers can be combined to create a complete Riven.
Requiem Mods have limited payloads, with three each, and one runs out when you kill a Kuva Lich with it. When all fees are used up, the Mod can be broken down into Endo.
Lohk
From the chasms that smolder we are seen
By the one without a name
Error
His heralds are the stars
Heaven and earth in flames
Flour
Bodily laws are not written
As the suns and love recede
Vome
To the laws of cosmic madness
Though uncompromising minds implore
Ris
In a luminous space blackened stars
They watch, accuse, deny
barrel
Roiling, moaning, our kingdom
In the lost madness will die
Netra
Hordes of coals trill their profane
Agreement with eldritch plans
Khra
Towards the cosmic forms of tangent planes
We end like we started