Truth and Trespass
Locations: Tempest, Havarl, Nexus, Elaaden, Kadara
Sometime after you've recovered the Salarian Ark, Kallo will ask for a moment of your time on the bridge.
He's received an anonymous message that the Salarian Ark may have been betrayed to the kett, rather than simply captured. There's a navpoint on Havarl for a meet. Follow it to the south end of the Chasm of the Builders, and find a kett ambush underway.
The kett forces are just Chosen and Anointed, though of course the wildlife may decide to join the brawl as well. When all hostiles have been neutralized, investigate the orange crate of Kett supplies.
This will trigger a convo with a retired STG officer, suspicious of an Dr. Aden having aided the kett. You'll get a Viability reward just for making it this far, and new navpoints on Voeld.
The simpler objective is scanning Aden's equipment in his lab at Techiix. There are many items to scan here, and you should scan them all, as they all yield research data. The plot important one, however, is the Angaran Data Core shaped device, which has suspicious chemical traces. Again, you can an odd mid-mission AVP reward for this progress.
Your other Voeld objective is checking out Aden's alleged Kett handler, but this really just means exterminating another outpost.
There are some high level enemies here, but no Ascendants or Fiends. Clear them out, then scan the handler body for a circumstantial clue, and yet another AVP reward.
With your leads dry, have a chat with the Salarian Pathfinder back on the Nexus, at Pathfinder HQ.
He'll suggest you search a certain apartment. This one is where all the apartments are- in the Docking Bay. Head inside and scan everything. Once you've accumulated enough data for further investigation, leave the apartment to run right into the owner.
Doesn't matter what you say to him, because you'll never see him again. Follow your data to Elaaden, and a downed shuttle. Use your scanner to reveal footprints leading away from the crash.
At the end of the trail are a passel of hostile raiders. (Note: there's a bug where it looks like there are just two raiders, but it's actually almost a dozen raiders, their bodies overlapping. You can't tell unti they split up and move in different directions, but just be careful about walking into more firepower than you expect).
After killing the multiple men, discover the real Saelen here- the other was Aden in disguise. It all seems a little needlessly complicated, intrigue-wise, but then they are Salarians.
Your final navpoint is a Kadaran cave (Kadara: Andromeda's Leading Provider of Subterranean Betrayals). Be sure to loot the container here before beginning the conversation- it disappears afterward
At the end of this dialog, you'll have the chance to either let Aden go free in exchange for intel, or let Saelen arrest him. The latter is the only option that really makes sense- if Aden is lying you've lost nothing, and if he's telling the truth that he did it to defeat the kett, then he'll likely give up the information anyway (or, as he says, it will have all been for nothing). Plus, if you arrest him, the real Saelen shows up to help in the final fight.
Whatever you choose, the mission ends here, with an XP reward, but no more Viability.