Well after several days of work on adding nihilanth to the monster_plugin I've come up short and am thinking there may just be too much going on and specialized for the circumstances set up by the map he's used in on the single player side to make it possible to be part of monster_plugin. I've actually got the code to compile, the monster plugin loads up fine under metamod but spawning him into game causes the server to crash. First there was the issues of the recharger crystals he expects to be on the walls to revive his health, remember you had to shoot those things out off the walls before you could kill this dude. So I managed a hack to sorta trick that part but then it crashes again. -debug mode of hlds tells me the crashing is going on in the NextActivity function of CMNihilanth and I've added in a bunch of debug output to console to try to see where things are going wrong but its just getting too frustrating as there seems to be so many things hinging on this monster that I could spend another week end feel I'd still come up short. If anyone wants to play around with this and look at the code I have it located at
http://bs-linux.com/nihilanth.cpp <- let me know if you want to see the other files ie cmbasemonster.h etc.
I think I'll go ahead and proceed with packaging up what I have minus nihilanth for now and see if anyone else has better luck than I.