Now some big mobs just attack u on sight, but i think some RP would be better before they attack you.
For example take the dark guard. This is a big(gest) perm around, but he attacks u on sight. But wouldn't it be better if he says something like:
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Who dares to disturb me in my crypt. Why didn't my guards stop you?
None who entered my crypt came out alive, and neither shall you! Now face my wrath!
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and after this he attacks you. Ofcourse you can attack him before that if you want. This way it makes the creatures, especially the big one, more alive.
Maybe it's also a good idea that when the first is allready started and he is around 50% he says something like 'You can never defeat me!' and when he kills someone he says (or broadcasts, depends on how big the perm is) 'Hahahaha, you thought you could beat me? Die!'. And when the mob is about to die he might say something that involves a secret, like a little hint about what to do with an item he drops.
EDIT:
about the dark guard.. not sure if he agro's on sight, but that's a small detail.