Warframe

Before even attempting this achievement you'll want to have completed The Deadlock Protocol, Rising Tides, Call of the Tempestarii and The War Within.

After doing so you'll want to head to any high level Corpus ship mission, I can recommend 'Hydra' on Pluto as it is a quick and easy Capture mission which is level 30+ allowing for you to farm Zenith Crowns at the same time and do this solo for simplicity reason, you'll also want to bring the Xoris you got in Deadlock Protocol.

If you're new to this then you'll need to collect a Granum Crown, specifically a Zenith Granum Crown. This specific Crown will drop after you kill a Treasurer inside of a level 30+ Corpus Ship node, these Corpus unit will spawn after an amount of time has passed, about 2-4 minutes (make sure you don't idle around).

After obtaining a Zenith Granum Crown you'll want to look for a Golden Hand and deposit your Granum Crown. Here you'll be required to kill 25 Errant Specters, make sure you do so otherwise a sister candidate will not spawn, if you brought the Xoris this should easy.

After exiting the void and reaching rank 1 (killing 25 Specters) a Sister Candidate should spawn in, you may have to wait a bit but it should spawn eventually. Locate and down the candidate, if it's the weapon you want then execute a parazon finisher to create your very own SIster of Parvos.

Once you've extracted you'll be able to check your Sister of Parvos just like you can with a Kuva Lich, you'll be able to see the Weakness, Resistance, Immunities, Weapon, Weapon element bonus which is determined by the frame progenitor and if applicable, their quirk.

Here you'll pretty much do the same thing you'd do when dealing with a Kuva Lich (skip this section if you've already done Kuva liches).


Once you've managed to guess all 3 Requiem Mods your Sister of Parvos will then flee to the Neptune Proxima, head over to the Railjack star chart and you'll notice a new mission in Neptune Proxima called "Sister of Parvos confrontation" I also recommend doing this on Public due to each Sister being eliminated will grant 1 Corrupted Holokeys and will make the mission easier overall.
Once you've entered the mission you'll need to do the objectives the game tells you before you can enter the capital ship like any other Corpus Railjack node.
Once you've enter the capital ship you'll be directed eliminate each party member's Sister of Parvos. Eventually your Sister of Parvos will show up, down her and make sure to Vanquish, NOT convert her so that you can get her weapon.
After eliminating all of the Sister of Parvos extract back to your Railjack and head back your Dojo and back to your orbiter, now head over to your foundry and claim your Tenet weapon and the Achievement should unlock.

If I missed anything or overlooked something please let me know!