Hi, good to hear from you again. This is certainly possible to add and would be a fun addition. I'd probably implement it something like this:
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A new "Nemesis" mod tab for enabling/disabling.
Starting at gate level 15 or so, the highest level gate has a skull symbol added to it.
Upon entry, a visually modified and scaled enemy begins targeting and moving towards the player.
It can pull from all available attacks and can swap between them during combat.
It has greatly enhanced combat stats to start and is also immune to stun/ragdoll.
Upon defeat, it receives improvements to combat stats, which is saved on a per save file basis.
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I can add it to the pack, though I am still finishing some other requests first (mostly model work). Let me know if the proposed implementation seems decent or if there is anything you would add or alter.