ARKANIS GATH
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He is wearing the belt of immie.
He is immune to lvl drain, hold, imprisonment, death, blindness, deafness, feeblemindess, and the dreaded CTRL-Y... and can NEVER go under 1hp
but he is NOT immune to closing doors... nor INT drain. SO you can kill him that way.
Unfortunatly he doesn't drop his sword (its undroppable...

) but even so it would ruin the game. It gives you -25 A.C (thats what I got with 3 dex and no other equipment) and kills anything in one go.