Credit to /u/IndianaCrash for tipping me off, view his thing here:
When Senna gains 100 Souls - Thresh's number appears to be irrelevant, as I gained 100+ Souls and the event never procced, plus Senna's 100 Souls is a pain to get - a sidequest appears.
This is Senna's description:
This is Thresh's:
https://i.imgur.com/m5icLAW.jpg
Upon killing Thresh as Senna (or presumably Lucian), Senna gains 40 AP and Armour:
https://i.imgur.com/SMkjD4V.jpg
When Thresh kills Senna, he gains 50 Range, 40 AD, and 30 Crit Chance.
https://i.imgur.com/AtWfLIe.jpg
Presumably he gains the same bonuses with a different description if he kills Lucian.
Thresh does not appear to lose his Souls when Senna is victorious, according to my friend who I tested with. I'd love to test this more thoroughly and see Lucian's interactions but it's a bit of a drag to get to.
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