After hours of of "down a thrall, mercy a thrall, down a thrall, mercy a thrall, down a thrall, mercy a thrall, end the mission, start again.", the player defeats their Lich and is ready to spawn another with a weapon they desire.
They enter a Grineer mission, complete the objective, find a larvling, down a larvling, check for the weapon ("Nope, not this one."), and leave.
They re-enter the Grineer mission, complete the objective, find a thrall, down a thrall, mercy a thrall--WHOOPS. That wasn't a thrall; that was a larvling. The player is now locked into fighting a Lich that they are most likely not happy with. How could this be avoided?
First and most obviously, the player could pay attention; at all times, the player could be fully aware of their actions. This solution would probably involve a lot of coffee, and might lose a percentage of the playerbase that plays Warframe to relax.
Another solution could be to make an effort to differentiate the actions of stabbing a larvling from stabbing a thrall. The end goal would be to make 90%+ of players never accidentally even call a larvling "thrall". One common suggestion would be to add hold-to-trigger to stabbing a larvling; this might leave the player in an awkward position in the middle of combat, but the player could learn to overcome that.
Another slightly less common solution would be to re-purpose the "convert/vanquish" screen from the end of a Lich's life and mirror it at the start of a Lich's "life"; this has the advantages of adding an opportunity to give invulnerability to player while they make their decision, and give players new to Liches the option to realize they are stumbling across an "unknown" part of the game they might want to forgo until they learn more, rather than just following the game's orders and stabbing the larvling when presented the option. Also, hey, the mirroring in the system is just elegant.
Another option is to change up the thralls rather than the larvlings. How necessary is the the stabbing of every single thrall, if the lore of the parazon's purpose isn't properly explained? Ok, ok, maybe that's going a bit far.
Nevertheless, the internal cycle of hunting the Lich could be, and I have no doubt will be, made less monotonous, tedious, and all around brain-numbing. I have no specific suggestions to this end at this time. The options are too numerous, and ideally would be intertwined with lore of the Lich program, lore which I do not know. These solutions are not mutually exclusive, and could be mixed and matched to some degree.
#number of times I had to correct "thrall" to "larvling" in this post: 1
(edit) I meant to include Lich cancelling as a potential solution for thoroughness' sake, but that obviously has issue with trivializing the hunt for ephemeras, and I don't much care for the idea systemwise or lorewise, personally, so I forgot to include it. Whoops.